Probably a bug in the game’s render state. I fixed a similar problem in Serious Sam 4.
You can either turn SK’s text OSD on (Ctrl+Shift+O) to prevent that from happening, or use an updated version.
Use the versions of the DLLs here:
Stop injection and just overwrite the DLLs that came with your version of SK and start it back up again. The DLLs might be locked, in which case just rename the old one.