Sending, receiving, and organizing email in Outlook.com
Normally, after going to Outlook.com > Settings > Mail > Forwarding and IMAP, enabling Let devices and apps use IMAP, and selecting Save, the setting should remain enabled. Since you mentioned this is for a new email address, could you please confirm:
How recently was this Microsoft account/email address created? Which browser are you using? Does the option switch back off immediately after clicking Save, or only after refreshing/signing out and back in?
As a test, please try signing in to Outlook.com using an InPrivate/Incognito window, enable IMAP again, click Save, close the Settings panel, and then sign out and back in before checking the setting again.