Part of the history can be lost sometimes

[rkfg]

I have no steps to reproduce it as it's quite random, so posting it just to gather some other confirmations if there any. So in a single week I found a couple of times that part of my timeline on the phone is missing. The last time it was about 18 minutes of conversation from 23:14 to 23:32. There were messages before and after the lost chunk and I only guessed something is wrong because some context of the conversation was missing. I checked the same room on the tablet and it was fine, the whole timeline has been present. There are several factors in play:

1. I have different accounts on the phone and tablet; the physical server that serves them is the same (VPS) though the domains are different and thus there are two synapse processes;
2. The one with the lost chunk is using a domain name delegated to a not quite good DNS server (name.com), I can't resolve it from time to time and had to put the IP to /etc/hosts on my PC. So maybe there were temporal issues in the federation, however, I checked the logs and there are federation send requests at that time so it was fine;
3. I took the phone out for some time so it should have switched from Wi-Fi to 4G and back (probably not a problem);

Still, after some time I scrolled back and the missing piece has been restored (the quicker way is to "clear cache" from the settings). I didn't find any errors in the logcat though. Also the same issue happened to another folk who's using a matrix.org account. Our room is also located on that unreliable domain but as I understand it that's only a name and the actual messages are posted to the homeserver and then replicated to other participating servers. So if he didn't see even the matrix.org-originated messages being on matrix.org himself there's something wrong.

As I said, if something like that happened to anybody please drop a line. The issue being that frequent is pretty critical but I'm not sure if it's common or there's something on my end. Anyway, what happens if a back paginate request fails for some reason? Does the user get some visual feedback or is it completely silent? Does it retry getting that segment or just moves to the next?