One of the restrictions of Outlook is to prevent users from saving draft emails in folders other than the default ones.
The consequence is that all draft emails are saved in default folders also when IMAP Accounts (like a Gmail account) are used.
However, users might want draft emails to be saved in IMAP draft folders so as to recover them from another computer or device like their phone. Unfortunately, this is not possible and users are therefore forced to copy or move manually draft emails with the risk that they forget to do that.
To overcome this limitation, you can install the Add-in OutlookDraftFolder which allows you to choose different draft folders where to save your draft emails, for multiple accounts.