Hi, I've got the same bug (macbook pro mid-2012, OXS 10.14.6 Mojave, basically, same model but different OS version). I originally posted my description of the problem here:
before I noticed I was dumb and that issue was already reported (for some reason, I only looked in the subforums when checking whether that was a known bug, as I said, I'm dumb)
I tried explicitly disabling (or enabling just in case, although I assume this does nothing) openGL4 (via the mkxp.json config) as suggested by andracass but it didn't seem to have any effect either.
[edit: here's an addition