No we did not change anything for a few months on these.
All was working as intended until recent updates from last week.
CBsubs related plan was set to hide the field also on registration if selected and that was the problem. Deselecting this field from the list of hidden ones by the plan for registration seems to solve it.
On the reverse, deactivating the condition on the field itself and reactivating the CBsubs plan hidden feature for the field does not seem to work as user has to select the field before choosing a plan, and selecting a plan by default brought other problems if I remember well.
Anyway, that's solved this way