PlatformIO not updating firmware from files/uploading previous version of file

Hello, I hope that I am able to accurately describe my issue.

I’m using PlatformIO with an STM32 blackpill board using the STM32duino core.

I have been using this configuration for a few months with no real issue, but recently this started happening: I will upload my code to the board for testing/debugging, make an alteration to the code, then when I upload again, the upload monitor will display the erase/download loading-bars and complete the upload, but the uploaded firmware will not have changed from the previous upload.

For example, if I have:


and the upload works (as it does occasionally, arbitrarily), if I then change the same line of code to say:


It will appear to upload the code like it is supposed to in the IDE, but when taking the MCU out of bootloader mode and reseting the board, the word “Hello” will still print to the monitor.

This began when I was deep into a project, and retracing my steps to when the behavior began did not ameliorate the issue, so I started an entirely new project. Eventually the behavior returned.

I have tried altering almost every part of my now un-postably large code to find the issue. I will post the entire multi-file code if necessary to find the issue.

Any help is appreciated, this is an extreme impediment in my process at the moment. Thank you.

EDIT: in case this is relevant to my issue, if I make corrections to pre-compiler errors on certain files (where there is a red underline say, if I forgot a semicolon, etc.), the red underline does not disappear and the tab at the top of the file still flags an error, even though the file (occasionally, arbitrarily) compiles without error.