The cooling back down to normal temps may be a result of thermal throttling.
What cooler are you using?
Probably worth removing the cooler, inspecting the thermal paste spread that you got on your first application, then re-pasting and remount the cooler.
While you've got the CPU cleaned off, do a dry fit of the cooler (without paste). Tighten everything down and make sure you can't wiggle the cooler on the CPU. I had this (mounting pressure) issue with my 5600G and Asetek AIO and [first] AM4 bracket. The bolt posts on the bracket were a millimeter too long and the cooler basically had zero pressure against the CPU.