In our developer account, a couple of the provisioning profiles have been listed status invalid. However, our App users are unaffected by this. We are not confident of what invalid profiles can bring if we leave them untreated. Is there anything we should expect can happen?
No user-facing effect, so don't worry
Those are only used when submitting to the store to prove that you are you to Apple.
If you later decide to update that app, those will need to be refreshed, however. Current Xcode should help with that.
I am curious as to the ad-hoc/enterprise answer here. A newly refreshed (in December) profile of ours has been marked invalid and we are noticing iOS 12 devices in the field start to display inability to download/install/launch with a new version we have released. It is unclear if the invalid mark occurred before or after the signing of the new version, although I would have expected a full-out failure if it was invalid at the time.
Strangely, Xcode does not seem to show the invalid state in its little info button/popup that shows the profile expiration and capabilities (this is after a download command on all latest profiles and a restart of xcode).