Hello - thanks for letting me know and sorry to hear you're encountering these issues! Unfortunately this likely is an issue with the interpreter itself, which is closed source; at the time I published the game, it was the only known interpreter for MSX, and Tristam Island is probably the largest game it's ever seen, which could explain issues.
Since the game came out another interpreter has come out: https://sijnstra.itch.io/vezza This one is open source, nicely optimized, and under active development so it might be the best option to play. I will update the MSX versions to point at this one instead.
Again, sorry to hear that and thanks for letting me know!