I'm stunned this hasn't been more widely reported. I've had this problem since September 2021, and I've found it extremely easy to reproduce on both the Linux "native" client and the "teams.microsoft.com" web client (with virtually any browser -- Firefox, Chrome or Edge). Just create a Team with two channels, add a 20-reply conversation to one channel, switch to the second channel and back to the first, and the collapsed conversation will now only expand to the last two replies.
I reported it to our local IT folks in October, but it was triaged as a "known issue" and never got fixed. I re-reported it to Microsoft last week, and they've been reasonably responsive and claim the back-end team has acknowledged the issue and is investigating. In the meantime, I've written a user script to hot-patch Teams when running under Firefox or Chrome (tested with Firefox 97 using Greasemonkey and Chrome 98 using ViolentMonkey). The script is available here: https://greasyfork.org/en/scripts/441058-microsoft-teams-patch-missing-conversations It fixes the problem for me (both truncated conversations and the blurred/loading state of old threads). Unfortunately, I don't have a comparable fix for the Linux client.