@emrl this would take quite some time to recreate. First I’d need to set up a site using ACF 5.4.X and create a test groups, add content and then update to 5.5 to see what happens. We’re talking about hours of debugging. Anyone having a similar issue should definitely be opening a new support ticket since this is more that I can do to help on this forum https://support.advancedcustomfields.com/new-ticket/
@litemotiv your first problem sounds like this one, but you’ve compounded the problem my rebuilding the field group. The reason that it’s no longer displaying on the front end of the site is that when you rebuilt it all of the field keys were changed to new one. It was originally working correctly on the front end because the field data and field keys remained intact. In order to recover at this point you either need to have a backup of your site or you’ll need to edit every post and update them so that the field keys for each field gets updated.
For anyone that took the time to back up their database before updating I would suggest that revert to ACF 5.4.X
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