Seemed recently purchasing my monitor or AMDs latest update all coming about the same time in noticing the issue -
Monitor set to DP v1.2 - LG 27UD68-W: 27 Class 4K UHD IPS LED Monitor (27 Diagonal) | LG USA
When toggling the DP 1.2 option on the monitor and going into display adapter Windows 10 i can somehow through toggling those 2 things repeatedly it comes to a 60hz own its from from the 30hz and remains ok until i restart.
Running with a SAPPHIRE Technology - Sapphire Nitro Radeon R9 390X & AMD FX8350
I have a 4k HDMI port on my TV that gives me issues as well. Have you tried changing to a different DP port? If yes, does it repeat the issue or does it work properly now? If it repeats the issue have you tried a new DP cable?
Edit: I meant to change to DP for your case but saw the HDMI I typed and ran with it.