Did you notice what the temp was before your disassembled?
Thermal paste should be cleaned off every time you remove the heatsink/fan/cooling solution from the top of the CPU. Then re-applied when you re-assemble.
It can be done with alcohol pads (after unplugging and de-energizing your PC) or something that doesn't leave behind a reside or fibers.
Q-tips and makeup-remover pads (not mine, lol)...with a dab of alcohol on them help take off the old paste very easily. And then clean it with a microfiber cloth to remove any reside or fibers. Blow it out with compressed air.
The old thermal paste, when you remove and re-assemble without cleaning/re-applying, can still have little air-bubbles in it that causes heat to not be transferred as easily and build up in certain areas. Which can cause what your describing.
Id remove the heatsink then clean the old paste (also notice the little indentations and uneven waves on the surface of the old paste, and any gaps)…….of course after disconnecting all cables from your computer and holding down your power button or a few seconds to disperse all the power.
Re-apply new paste.
And see if the issue still occurs.