Boot up OC drop???

Basics:

ASUS Z390-A MOBO

i5-9700 uP

CLK 3.4

Water cooled with avg temps around 60C

Problem:

On boot up I was getting a consistent 42% OC notice then last week is just dropped down to 27% on boot up.

Any idea what would cause this drop - out of the blue?

Thanks

Harry