The ROM's format is .gbc so it should've been recognized as such, same with the web version. Other than that, I have no idea why it does that.
118 days ago (+1)
Found the problem. you had Unit Code (SGB Flag) located in address 0146 to 03, which made some emulators that runs off of the ROM Information assume it was running a Super Game Boy ROM... And since the game is only meant for GBC, it assumes that it's running on ye ole gameboy. Changing 0146 from 03 to 00 fixes the problem.
How I found it out? Ran a comparison between this and a G-Zero World (which is also a homebrew) to eliminate as many factors as possible, and Unit Type (see image) quickly caught my attention. A little research in GB/GBC ROM Header showed quickly what the problem was.
118 days ago(+1)
I've originally planned this fangame to support all Gameboy models until I've realized that its poor performance on both DMG and Super Gameboy models wouldn't be an enjoyable experience, so I've added a lock-out screen & Super Gameboy border to highlight the fact. Since some emulators allow Super Gameboy borders to be displayed while playing in GBC mode, I believe that's where the issue is coming from.
A true fix for this is to either port the fangame to GB Studio 4 where the ROM can truly be set in "color only", or remove the Super Gameboy border altogether. (Editing the ROM's code may also work, though I'm not sure how as it's built from GB Studio's own ROM exporting option.)
Maybe I'll make a somewhat updated version of this fangame that fixes this issue, but that'll be for another day. Thanks for the help though!