Dangit! I really need to invest in some proper hardware to test these projects on. :(
Must be something to do with setting the 8x8px character graphics mode.
I'm glad you like the sight mechanic! I'll get some real hardware set up and get it fixed.
Plus it’s much more satisfying running what you wrote on real hardware :)
I have an alternative to suggest however. PCem is a very accurate emulator, and indeed I just tried your game in PCem and exhibits exactly the same issue as the real computer does. At the start of this jam a couple of months back, I posted in the “community” section, a handy archive including a PCem disk image with DOS 6.22 and a bunch of devtools pre-installed, as well as a PCem binary I compiled for GNU/Linux: https://itch.io/jam/dos-game-jam-2/topic/692522/disk-image-with-dos-devtools
I've still a lot to learn about DOS development! Weirdly, it works fine on a FreeDOS machine I've got, but not on Win98 and PCem shows up the problem for me too.
Thanks for the offer of help - I'll try stripping it back to narrow down the problem, and pull together a larger collection test environments, starting with your PCem tools!