Congrats on your game - it's no easy feat to ship a game for DOS. And you managed to keep a very good file size!
Did some tests on my machines:
486DX 50Mhz 12MB RAM = runs a little laggy, but playable.
486SX 33Mhz 8MB RAM = crashes at startup due to the lack of math co-processor.
Since Lua Number is a 64-bit floating point, your game will require floating point emulator (if you're using DJGPP, add -lemu at your linker command line).
This is also most likely the reason for the lag on my 486DX and also your known bug. But hey, there is a way around it! If you want, I can help you change Lua Number into a 32-bit integer (and most stuff will still work).
My lastest game, The Mistral Report (https://montyontherun.itch.io/the-mistral-report) also uses Lua (5.2), which I modified to use (almost) no float. At the end of the day, I had to add -lemu, but it's perfectly playable on the 486SX)