Well, I paraphrased the questions that de0u highlighted, and sent them in an email to ProtonVPN support. I now received the following reply:
Hello,
Thank you for your reply and the additional information that you've gathered; it has also been forwarded to our Android developers.
With all the information so far, we have no reason to believe that this is an exploitable vulnerability, but we can't guarantee this as the issue is still under investigation.
The Android development team is already aware of some crashes happening with WireGuard due to memory corruption, and they're actively being looked into.
However, I unfortunately can't give you a specific timeframe for when this one could be fixed.
We appreciate your understanding.
They did not tell me why they have "no reason to believe that this is an exploitable vulnerability", so I've asked them to clarify that.