Same here.
I used a Nokia 5.1 plus with Android 10.
Always latest Outlook app version, with 1 MS mail account and 2 accounts from a local provider called Swisscom. Accounts are setup over IMAP. All mail accounts on the servers are almost empty because i archive the mails to local folders on a laptop.
Outlook calendar is in use.
Problem started around November if I remember well.At 6GB the small memory of the Nokia was full and created corrupted memory issues. More than 4 factory resets. I thought about a hardware issue first. Then I localized the problem at Outlook, as described in this thread by many others.
Memory filled up mostly overnight at home while charging the phone battery.
When Android started with error messages, I uninstalled Outlook and reinstalled it several times.
Since a couple of days, I have a Google Pixel 4a with Android 11. After one night on charger Outlook user data reached 16GB! But in the Wi-Fi usage statistic Outlook used "only" 898MB.Microsoft, please fix this annoying bug in Outlook for Android app.
Hi, Nice to hear that we on this thread are not alone. Your symptoms are exactly the same as mine. I have a Pixel 64GB. I am not sure if the charging (yes or no) is relevant but it does seem to always happen at night when there is otherwise no activity: User Storage jumps from under 1 GB up to 16 GB! No doubt the Microsft fairies playing around - or the NSA scanning our emails. It is perhaps the combination with IMAP - who knows? I have found that you do not have to reinstall Outlook. Just delete User Storage (takes some minutes to get down to zero). Then when you start Outlook you have to setup your accounts again. With luck, it may remember the IMAP port settings so you do not need to enter them again.
Anyway - Microsoft does not seem to care that Outlook is devouring storage for no reason. It is a real pain. As you say, it seemed to start about last November. For the previous several years I have been using Outlook with IMAP on my phone with no problems.