It looks like an error also occurring yesterday in the United Kingdom environment. It seems that Exact Online is currently making changes to the integration of Azure B2C with Exact Online itself.
In case the problem reappears please update the topic.
The problem now consistently appears in all Exact Online countries. It seems like an undocumented change has been taken into production. An analyst will study the issue and try find a solution.
A number of improvements are currently internally undergoing tests.
The actual behaviour and/or problems have been found to vary per Exact Online country. It is currently unknown why Exact Online has different behaviour per country.