So I can tell you there are no settings for it. What compressed the conversations is leaving a channel and coming back to it. Least that’s what used to cause it and you could reproduce it every time. Not sure if anything has changed recently thou. Haven’t noticed anything.
Have attached a screen shot of what I mean by conversation.
Think I have figured out the pattern at least. It seems the up to 10ish days the first and last message will show (unless you have clicked expand so all shows).
When you get over this number of days it will just show the first message and the number of replies beneath it, so you can expand if you want. Unless there is only one reply then it will show the first and last (only) reply.
Had a user discussing/complaining that it doesn't always show the last reply to the string as they like to put a 'closing message' at the end of completed conversation.
It does highlight that it is a shame you can not put a status/colour code to each conversation for sorting within a channel.