This might not be useful so keep that in mind - this especially won't help track any memory usage problems.
I also converted a large test project from Mockito 2.x to 3.7.
We also used Powermockito and after a few weeks of trying to get both tools to work together, I decided it would be easier to just deprecate powermockito at the same time. To be clear, I was not seeing any memory errors: just errors such as ClassNotFound and NPEs.
I could get past some of these errors by changing the order of the dependencies in the POM, but that felt very fragile and didn't seem like it was addressing the root cause of the two tools conflicting. Even once I had the existing suite working, any new usage of Mockito would cause the previous errors to resurface. Instead of trying to track down why the two frameworks did not work together, I decided to remove PowerMockito at the same time as updating to Mockito 3.7.7
This actually worked and moved us to (just) Mockito 3.7.7. The advantages were:
1. we now have just one mocking framework to use and support
2. the conversion got done much more quickly (than if I had spent time trying to track down the conflicts)
Again, this is a little negative and doesn't benefit isolating any defects in mockito or powermockito. I just thought I would share what I had to do to get a similar update completed.