When I use the built-in contact app with either a outlook.com or a office365 account on the phone created contacts get deleted after some time and they don't sync to the web.
What have I done:
Similiar to https://forums.oneplus.net/threads/contacts-keep-deleating.484478/ except that I already save to another account.
I was unable to stop the outlook App overriding/deleting created contacts. But syncing contacts via gmail, but using the Outlook App for everything else seems to work.
Disable Contact syncing for all accounts added to the Outlook App in the system settings.
Add all accounts to as an exchange account to the google App (In Account settings, there should be an 'Exchange' option. Disable all syncing but contact sycning.
Q & A