Chat status said 'Sending...' it never sent, next day those chats are gone from the feed
Currently if a message in chat fails to send (desktop) it had a status of 'Sending...'
This remained like that until I left the office.
Later the same day I check my app (mobile) and the messages that were sending are not shown on screen at all.
The next day the app (desktop) no longer shows these pending messages at all.
Conclusion: You are unable to see messages that remained unsent, they seem to be deleted/removed from private chat. Problem there is you don't know the status. If a message fails to be sent in any chat it should be marked read, as failed and the user who attempted to send should have some form of notification.
Kim Hyungki commented
I have experience. Teams Mobile Chat often fails to send messges.