Search field filter does not work

Dear colleagues,

I have a strange issue in an ERPNext v10 instance: we two custom doctypes, one of them (SDG Goal) is linked into a field of the other. On SDG Goal, search fields are title and nr. This shows correctly on the form:
However, if a part of the key (e.g. 5) is entered, the list goes empty.

There is no query override. What am I missing? Thanks!