I had the similar problem with my ASRock Z270 Supercarrier mobo and 7700K. It wouldn't drop that low, but It would drop from 5.3GHz to 4.9GHz. I had to set and AVX offset of 1 to get it to go from 5.3GHz to 5.2GHz. It would always take the Cache frequency down to 4.9GHz no matter what I did though. After a newer BIOS dropped (p2.20), I didnt have to use an AVX offset and it would stay at 5.3GHz, however, there is nothing I can do to keep the Cache frequency to at the same speed as the cores. For example:
Under water the Cache frequency drops from 5.3GHz to 4.9GHz.
Under Dice the Cache frequency drops from 5.8GHz to 5.5GHz.
It you hear anything about this issue please let me know yos.