All our users are enforced on Office 365 built-in MFA. But Flow connections seem to invalidate after some time. Especially when using Forms as a trigger, combined with "Apply to Each". No errors are shown in the flow, still the flow sometimes hangs indefinitely on Apply to Each. When running under a non-MFA enabled account, this does not happen.
Please improve the Flow service, and make connections unaware of MFA. Flows are basically 'automatic runs' in the background, running on MS servers. Therefore these runs don’t require MFA verification, in my opinion. Except at creation time, of course.