I changed the title, and I add this.
Finally, the only types that seems to work on the “search” endpoint, are:
enum
text
number.
ALL other types of custom fields are ignored using the “is_set” filter.
we tried with dates, people, multi-enum.
All custom fields from that post, and added after, are non searchable.
Again, I understand that filtering by value can be difficult to implement, but the simple “is_set” should work for any kind of custom field.