I looked into the code and I can't find anything thing that can cause jump to go smaller. I tried older version of Love2d (ver10.2) and it works fine. Linux also has no issue on jumping. Seems like an issue on Mac only, and unfortunately I do not have a Mac to test it :(
But feel free to look into the source code and see if you can find the issue. The section that controls the jump is in between line 361 and line 380 in PlayState.lua. It is a tiny section of code.
Edit: Released a possible fix! :)