Should be fixed in the upcoming version, you can test the changes in the current upstream source. I changed the font visualization in C64 mode so that it uses standard-sized letters - sadly, it means a little less text can be fit into things, but at least it doesn't cause weird side effects because the downscaled letters are sub-tile size
If everything works fine, I'll release a new version tomorrow with these fixes in.
Thanks a lot for the input, guys! I wish you all a happy New Year!
UPDATE (12/31/21): Version 1.1.4 has been released. It's a bugfix/polish release that addresses the following issues:
* Fixed scrolling in intermission caves in C64 mode. Now intermission caves do not scroll and only show the top left corner of the cave.
* Improved visualization of text and popups in C64 mode, now the font is larger, so there's less allowed amount of text, but there are no glitches with sub-tile rendering (oddly colored tiles, gaps between letters, etc.)
* Fixed the text centering routine which in some cases broke through the boundary of the popup window (will now break text mid-word if necessary in order to stay within bounds).