For people not satisfied with the reliability:

Me too, at the beginning.
But I have never had a problem when it is Android to Android.

After tinkering for a long while I found out that:
The connection is much more reliable after allowing mDNS in my firewall settings.

I don't know if this is related. If it is, I hope it was mentioned somewhere so users don't get frustrated when the app doesn't work.

In my OS, mDNS is blocked by default for public networks. (and every network is a public network by default.)
The firewall is preinstalled (firewalld); the silver lining is that I understand more about firewalld and appreciate its customizability.
Before this, I was only able to use ufw.