de0u Pocketcast and Syncthing are the two apps I'm experiencing this issue with. Syncthing from F-Droid and Pocketcasts through Google Play Store.
I've only ever connected to the one WiFi network with this phone, and I've done several phone reboots and a WiFi box restart yesterday. Suppose I could try connecting off my partners hotspot or something.
Good point on the VPN. I use DuckDuckGo's App tracking protection which uses a VPN for its function.
I had previously turned it off for just the apps having the issue, but have now turned it off for all apps.
After a reboot, Syncthing and Pocketcasts are working as expected.
Curiously, I've turned the App Tracking Protection feature back on in DudkDuckGo and both apps are working as they were before...
If it was the VPN, I would have expected that to also be restarted between phone reboots?
Guess I can chalk this up as solved.