Just want to say engine designers had been using thermo imagery to detect hot spots in engines for a long time. If the engineers who desinged this engine haven't utilize that as one of their tools, Toyota should get rid of them! Enough said, back to work at hand!
Anyway, we are here to help
@Alexqkama to determine what is wrong with the engine at hand, not speculate on what may or may not be the cause. If you want to provide insight, excellent, the more the merrier!
Have yet to determine what kind of fluid is on the plug yet, since OP have not confirm what the fluid is or feel like! So let alone what exactly is causing the issue.
When OP return with the issue, mentioned about bubbles in the cooling system, (didn't go back to reread the post, so maybe off) but now doesn't seem to have bubbles anymore, at least didn't show at low pressure leak down test. Not sure when high pressure is used for the test! Wait for OP!!!!
Not here to argue or debate with anyone, and have to admit not always right with the conclusions, after all not being there in person makes a difference on how to diagnoise an issue! A question not asked can at times make the differences between right or wrong on the conclusion! Don't know everything, still learning everyday!

Just saying!