Group as memberOf dynamic group no longer working

Question

I've a number of dynamic groups that make use of the nested memberof properties of other groups. These were set up a year or two ago.

I've come to modify them (just removing one of the nested group objectIDs from the rule) and yet this will not save as "wrong property applied".

Have tried recreating the rule using the instructions on the related learn article but still won't validate the rule check.

Has something changed/broken?

https://learn.microsoft.com/en-us/azure/active-directory/enterprise-users/groups-dynamic-rule-member-of

Edit: I've just gone to check, and the last time I was able to create a dynamic group using this rule was on 12/09/23 @ 22:00, so only a couple of days before hand. Some change implemented within the last week