not sure about the wintun thing, i know it's suppose to be faster, but i'm using regular "tap" adapter and i get same speed with or without vpn just with higher latency, but i'm also only on a 180Mb connection... but if you are having issues but as far as the speed goes there's a sticky on page one i was just reading a few min ago and it was looking like there could be some isp's that are throttling vpn traffic sometimes? but there was a fix in that thread that seemed to help a few people out. Look on the first page of the troubleshooting section for a stickied entry titled " Every VPN is slow for me, despite the well-reviewed VPNs I'm trying. Is it possible my ISP is causing this? ... "
also like you, i'm on win 10, and every once in a while i'll get that same thing with a "no internet" symbol in the task bar, but internet still works. but mine is usually when bringing the computer out of standby, don't think i've ever had it happen when disconnecting from air, but i'm also not using wintun, not sure if that's anything to do with it, but i do know it's a lot newer tech. but just like you if i restart it goes back to normal operation so its prob a windows bug and not an "air" problem. because i've seen it happen randomly on other pc's too that aren't running vpn.
edit: just found this link in another thread talking about how to fix the "no internet" bug on win 10, if this doesn't help check out the thread on it here in troubleshooting. https://www.windowslatest.com/2020/07/18/windows-10-no-internet-connection-problem/