There was a unfortunate bug in the Modeler of Mendix 5.1.0: access rights of access rules were reset to their default values when making certain changes in the domain model, like adding or removing attributes.
If you have made domain model changes in 5.1.0, we advise you to revert to the situation before making those changes and apply them again.
We have removed 5.1.0 and quickly released 5.1.1 that contains the fix for this issue:
We sincerely apologize for the inconvenience.
We just had a communcation with Mendix. There are indeed issues with this release. There will be a new one today or tomorrow.