Unexpected behavior from
my customized Marlin firmware

I modified Marlin config files, successfully built a binary and flashed it, but the firmware didn't react my modifications or behaved strangely.

Some of your modifications were not compiled into object files and thus the final firmware binary.

Close your IDE (eg. VSCode, Arduino, etc.) to prevent any auto compile actions.
Delete the .pio directory under Marlin directory. All compiled files are there. The .pio might be hidden as the name begins with a dot.
Open your IDE and build again. Try this clean built firmware binary.