I confirm that Poweramp does a license check not only after it is reinstalled, after a system reset (whatever that means), or after a ROM update.
My wife and I are both Poweramp users. I convinced her to purchase the app because I was very satisfied with Poweramp. However, it appears that she was never able to use the app for an entire flight. After some time with the phone in airplane mode, the music player walks out on her saying that the license verification failed. This is not reasonable at all and makes Poweramp look like a scam.
The strange thing is that the failing license checks happen to my wife, but not to me! And our phones are identical: each of us has a stock Galaxy S7. In both phones, Poweramp shows the information "Purchase Verified" and "Store: Play" in Settings->About.
A difference that may or may not be relevant is the app usage pattern. I use Poweramp most of the time. My wife is more like a casual user, who sometimes spends several days without opening Poweramp. But she wants to use it when traveling by plane or by car. And the app in her phone refuses to work in the absence of an internet connection... Yesterday we traveled together, side by side in a plane. I used Poweramp during all flight, but she got a failed license check after half an hour or so.
If the intention is for the license to be "checked once/twice (website/play) and stored forever", then Poweramp has a bug. The problem may not be easy to understand and reproduce, but it exists and shows up in a stock Galaxy S7. If the developer cannot fix the bug, he should at least provide some workaround for the unlucky paying customers that are being bitten by it.