Maybe set up a script that runs locally and pings an external service like 1.1.1.1 or 8.8.8.8 every second to see if it survives in a window when your services alert? Perhaps it's your modem refreshing some config which causes a blip for a few seconds or something similar. If this doesn't alert at least you can rule out that your internet fully goes out.

The other side of this would also be useful, if you could run a similar check towards different levels of your home network to see how far down it gets (e.g. ping your router, expose some simple TCP echo service on the server running all this and nc it, curl the status page of the reverse proxy (or set up a static page in it), curl the app behind the reverse proxy - just make sure to use firewall rules for this and not just put everything on the internet). Depending on where it fails should hopefully give you some idea to go on.

Maybe set up https://www.thinkbroadband.com/broadband/monitoring/quality/ to see if it registers any packet loss in those times or increased latency (although I'd still do the above as well)