For the records: I just had the same issue, neither /api/contacts?minimal=true nor /api/contacts?select=id,email preventing the huge arrays being fed back.
I do think there’s some buggyness.
BUT here are two nice workarounds:
A) minimal=true&select=id DOES prevent the flood, returns a rather slim amount of data
Example:
/api/contacts?minimal=true&select=id
{"total":"662060","contacts":{"2911715":{"isPublished":true,"dateAdded":"2026-05-29T14:05:23+00:00","id":2911715,"points":0,"fields":{"core":[],"social":[],"personal":[],"professional":[],"all":{"id":"2911715"}}}}}
–
B) syntax like minimal=true&select%5B%5D=id&select%5B%5D=firstname&select%5B%5D=email even allows you to select additional fields and still stay lean 
Example:
/api/contacts?minimal=true&select%5B%5D=id&select%5B%5D=firstname&select%5B%5D=email
{"total":"662060","contacts":{"2911715":{"isPublished":true,"dateAdded":"2026-05-29T14:05:23+00:00","dateModified":"2026-05-29T14:41:14+00:00","modifiedBy":2,"modifiedByUser":"Ekkehard G\u00fcmbel","id":2911715,"points":0,"fields":{"core":{"firstname":{"id":"2","label":"First Name","alias":"firstname","type":"text","group":"core","object":"lead","is_fixed":"1","properties":"a:0:{}","value":"Martina","normalizedValue":"Martina"},"email":{"id":"6","label":"Email","alias":"email","type":"email","group":"core","object":"lead","is_fixed":"1","properties":"a:0:{}","value":"test@test.de","normalizedValue":"test@test.de"}},"social":[],"personal":[],"professional":[],"all":{"id":"2911715","firstname":"Martina","email":"test@test.de"}}}}}