Have you made sure to modify the board before using it?
This page mentions at least one resistor that you have to replace for the USB to "work".
The USB standard requires a 1.5 kΩ pullup resistor on D+, but this board is known to have a wrong value (R10 on the board). It ships with either a 10 kΩ resistor or a 4.7 kΩ resistor, but it should be replaced with a 1.5 kΩ resistor, or put an appropriate resistor value (e.g 1.8 kΩ) in between PA12 and 3.3V.
It is also true that some PCs are tolerant of incorrect value so, before you change the resistance, you can try if it works in your case.
I have had issues with bluepill boards myself in the past, but never had an issue like yours after fixing the resistor. (before that, I couldn't even flash the bootloader)
Try printing other things with the board, and see if there's any pattern to it cutting out. e.g. a string over and over again, or the same float over and over again.
Try different baudrates, or even SoftwareSerial to another board.