https://bugs.freedesktop.org/show_bug.cgi?id=107978
--- Comment #14 from Shmerl shtetldik@gmail.com --- (In reply to Nicholas Kazlauskas from comment #13)
If I had to guess I would say this is an issue specific to that monitor - DP 1.2 should generally work without issue.
It did work fine before in DP 1.2 mode (like with kernel 4.17.x), so there must have been some regression which caused this behavior.