I'm also having this problem with the 1903 update. I rolled back to 1809 and my overclock came back. This happened in 1809 update but a microcode update applied to the bios corrected the problem. For my 6800K this was version B00002E. This update installed a new mcupdate_GenuineIntel.dll timestamp is 3/18/19. Previous mcupdate_GenuineIntel.dll timestamp was 9/15/18.