M365 disconnects my Dropbox account every single day

It appears I've solved this by adding Dropbox to the Storage settings under my profile in each MS 365 app. Doing that has allowed the Dropbox connection to become persistent again; no more need to add it each time I go into a MS 365 app. 

(I should probably add that I did what follows on my iPhone. The interface and, potentially, process may be different on an iPad.)

To do that, open each app, click your profile -- likely your initials -- in the upper screen, then Settings, then Storage Accounts, then Add Storage Account ... then (re)connect Dropbox.

I have no idea why the persistent connection went away to begin with. But doing the above allowed it to remain on all platforms for me. And sometimes, all's well that (just) ends. 😉