This comment won't help you with your issue but comment on:
Also, how did you make a Linux version when the original game is only available on Windows and MacOS.
Basically, UNDERTALE and DELTARUNE are both games made by Toby Fox. DELTATRAVELER however is an unofficial fangame made by a separate team of people who make games in a differing way to Toby. (Also according to another reply, UNDERTALE does have a Linux version) The game was most likely also developed within different tools (which well actually is the case, UNDERTALE is made in GameMaker while DELTATRAVELER is made in Unity.
But basically, the games have support for differing platforms due as they are two seperate games.
(First time replying/commenting on itch. Hopefully not too lengthy.)