Inconsistant compilation error

Question

It is s a bit complicated to ask such question, but what may be the reason that one time compilation passes and other fails without any changes in code? surely debugging this way is very hard (one time commenting out one part of code seems to stop this problem and later it returns even if commenting out was canceled).

I suspect it has to do with some changes in code regarding some ArduinoJson functionality

Sketch was was tested on 2 different platforms, Linux and Mac, and error occurs on both.

What troubles me is compilation inconsistentancy rather than some bugs in my code.

Error message:

/home/guy/snap/arduino/70/.arduino15/packages/esp8266/tools/xtensa-lx106-elf-gcc/3.0.4-gcc10.3-1757bed/bin/../lib/gcc/xtensa-lx106-elf/10.3.0/../../../../xtensa-lx106-elf/bin/ld: sketch/inFlash_Parameters.ino.cpp.o: in function `_Z17updateTopicsFlashRN18ArduinoJson6171_9112JsonDocumentE':
/home/guy/Documents/Dropbox/Arduino/libraries/myIOT2/examples/inFlash_Parameters/myIOT_settings.h:46: undefined reference to `_ZN6myIOT223extract_JSON_from_flashEPcRN18ArduinoJson6171_9112JsonDocumentE'
/home/guy/snap/arduino/70/.arduino15/packages/esp8266/tools/xtensa-lx106-elf-gcc/3.0.4-gcc10.3-1757bed/bin/../lib/gcc/xtensa-lx106-elf/10.3.0/../../../../xtensa-lx106-elf/bin/ld: /home/guy/Documents/Dropbox/Arduino/libraries/myIOT2/examples/inFlash_Parameters/myIOT_settings.h:44: undefined reference to `_ZN6myIOT228update_vars_flash_parametersERN18ArduinoJson6171_9112JsonDocumentE'
/home/guy/snap/arduino/70/.arduino15/packages/esp8266/tools/xtensa-lx106-elf-gcc/3.0.4-gcc10.3-1757bed/bin/../lib/gcc/xtensa-lx106-elf/10.3.0/../../../../xtensa-lx106-elf/bin/ld: sketch/inFlash_Parameters.ino.cpp.o: in function `_ZN18ArduinoJson6171_9110MemoryPoolC4EPcj':
/home/guy/Documents/Dropbox/Arduino/libraries/myIOT2/examples/inFlash_Parameters/myIOT_settings.h:52: undefined reference to `_ZN6myIOT223extract_JSON_from_flashEPcRN18ArduinoJson6171_9112JsonDocumentE'
/home/guy/snap/arduino/70/.arduino15/packages/esp8266/tools/xtensa-lx106-elf-gcc/3.0.4-gcc10.3-1757bed/bin/../lib/gcc/xtensa-lx106-elf/10.3.0/../../../../xtensa-lx106-elf/bin/ld: sketch/inFlash_Parameters.ino.cpp.o: in function `_Z22readParameters_inFlashv':
/home/guy/Documents/Dropbox/Arduino/libraries/myIOT2/examples/inFlash_Parameters/myIOT_settings.h:52: undefined reference to `_ZN6myIOT228update_vars_flash_parametersERN18ArduinoJson6171_9112JsonDocumentE'
/home/guy/snap/arduino/70/.arduino15/packages/esp8266/tools/xtensa-lx106-elf-gcc/3.0.4-gcc10.3-1757bed/bin/../lib/gcc/xtensa-lx106-elf/10.3.0/../../../../xtensa-lx106-elf/bin/ld: /home/guy/Documents/Dropbox/Arduino/libraries/myIOT2/examples/inFlash_Parameters/myIOT_settings.h:57: undefined reference to `_ZN6myIOT223extract_JSON_from_flashEPcRN18ArduinoJson6171_9112JsonDocumentE'
/home/guy/snap/arduino/70/.arduino15/packages/esp8266/tools/xtensa-lx106-elf-gcc/3.0.4-gcc10.3-1757bed/bin/../lib/gcc/xtensa-lx106-elf/10.3.0/../../../../xtensa-lx106-elf/bin/ld: /home/guy/Documents/Dropbox/Arduino/libraries/myIOT2/examples/inFlash_Parameters/myIOT_settings.h:59: undefined reference to `_ZN6myIOT223extract_JSON_from_flashEPcRN18ArduinoJson6171_9112JsonDocumentE'
collect2: error: ld returned 1 exit status
Multiple libraries were found for "WiFiUdp.h"
 Used: /home/guy/snap/arduino/70/.arduino15/packages/esp8266/hardware/esp8266/3.0.2/libraries/ESP8266WiFi
 Not used: /home/guy/Documents/Dropbox/Arduino/libraries/arduino_648747
 Not used: /home/guy/Documents/Dropbox/Arduino/libraries/WiFiNINA
 Not used: /snap/arduino/70/libraries/WiFi
Multiple libraries were found for "ArduinoJson.h"
 Used: /home/guy/Documents/Dropbox/Arduino/libraries/arduino_215091
 Not used: /home/guy/Documents/Dropbox/Arduino/libraries/arduino_892158
exit status 1
Error compiling for board NodeMCU 1.0 (ESP-12E Module).

Sharing function that are mentioned in error to make the picture clearer:

void updateTopicsFlash(JsonDocument &DOC)
{
    for (uint8_t i = 0; i < 3; i++)
    {
        sprintf(topics_gen_pub[i], DOC["pub_gen_topics"][i]);
    }
    for (uint8_t i = 0; i < 3; i++)
    {
        sprintf(topics_pub[i], DOC["pub_topics"][i]);
    }
    for (uint8_t i = 0; i < 2; i++)
    {
        sprintf(topics_sub[i], DOC["sub_topics"][i]);
    }
}


void readParameters_inFlash()
{
    StaticJsonDocument<1250> DOC;

    iot.extract_JSON_from_flash(parameterFiles[0], DOC);
    iot.update_vars_flash_parameters(DOC);
    DOC.clear();

    iot.extract_JSON_from_flash(parameterFiles[1], DOC);
    updateTopicsFlash(DOC);
    DOC.clear();

    iot.extract_JSON_from_flash(parameterFiles[2], DOC);
    DOC.clear();
}

calling 2 functions from lib:

bool myIOT2::extract_JSON_from_flash(char *filename, JsonDocument &DOC)
{
    File readFile = LITFS.open(filename, "r");
    DeserializationError error = deserializeJson(DOC, readFile);
    readFile.close();

    if (error)
    {
        return 0;
    }
    else
    {
        return 1;
    }
}
void myIOT2::update_vars_flash_parameters(JsonDocument &DOC)
{
    useWDT = DOC["useWDT"];
    useOTA = DOC["useOTA"];
    useSerial = DOC["useSerial"];
    useFlashP = DOC["useFlashP"];
    useDebug = DOC["useDebugLog"];
    debug_level = DOC["debug_level"];
    useResetKeeper = DOC["useResetKeeper"];
    useNetworkReset = DOC["useNetworkReset"];
    noNetwork_reset = DOC["noNetwork_reset"];
    useBootClockLog = DOC["useBootClockLog"];
    ignore_boot_msg = DOC["ignore_boot_msg"];
}