Additionally, group chats often split in to two windows where the conversation suddenly continues in a second thread even when no one joined/left... and then you try and reply and it jumps to another conversation on its own.
People in my office have recorded this because it's so obtuse and unbelievable.
I used to use Pidgin with the SIPE plugin, which worked well enough, and most importantly, logged in plain HTML with a fully working search mechanism, and if it came to it, I could just grep the folder.
Unfortunately my company recently moved to cloud-hosted S4B, and that completely broke Pidgin, so I've been forced onto the official client.
Someone at Microsoft really needs to learn the meaning of 'improvement'.
Assuming you're in a corp environment, that's likely an IT decision, not a MS one. If you go to Options -> Personal there are two checkboxes for saving IM conversations and call logs in your Conversation History folder in Outlook. Mine are grayed out because IT decided to disable them (I'm sure to meet some corp requirement).
Strange thing is that indeed, the "save conversations" option is grayed out (unckecked) in S4B, yet we are allowed to use Slack.