https://bugs.freedesktop.org/show_bug.cgi?id=110199
--- Comment #12 from jacobbrett+fd.o@jacobbrett.id.au --- (In reply to Bennet from comment #11)
Only kernel 4.19 works for me with 75hz , so probably seem to be a problem related to Free Sync.
I don't think it's Freesync-specific; I have Freesync turned off but suffer the same issue at 75 Hz; working fine at 60 Hz, though (RX 580).