I thought that this is an issue but obviously it's a "feature": Each time it is launched, Outlook automatically expands a non-POP account's folder list in the navigation pane.

See my posting here:
https://answers.microsoft.com/en-us/msoffice/forum/msofficeoutlook-msomac-mso_o365b/outlook-1537-170815-unwanted-account-expands-in/0d6527ac-e857-4e68-ae3c-47cfc38e4025?auth=1