I have major issues. Threads not marking as read. Syncing issues where I have missing messages. I have to kill the app to get them to load. Open push notification and do not see the message.

Could be a myriad of things, unfortunately. Have had my battery saver settings kill some background activity for example and adding Slack to the exception list fixed a lot of issues for me.

Maybe contacting the customer experience team could help. You can just use /feedback to open a ticket and they're pretty quick to respond.