Sometimes I accidentally click the group expansion/collapsing toggle button and the messages from that group get hidden. Of course, I can re-show them if I click the button again. But it gets weirder than that:
If after such a group collapsing/re-expanding I delete or move all messages in that group to other folders and eventually close the Outlook application, the next time I come back and there are newer messages to be shown for the same group, it gets loaded as collapsed (although it was left as expanded).
It took me some time to find a workaround for this apparently minor issue (actually it is kinda annoying): as soon as you inadvertently collapse a node, instead of re-expanding it (or after that), right click the group header and select Expand all (“Extindere totală” in my localized Office). Believe it or not, that will do the trick!