while querying the rest api with
http://localhost:28017/db/col/?filter__id=f2ce2c369e6660ba3b1397f0
gives back correctly the document saying
query: {
_id: "f2ce2c369e6660ba3b1397f0"
}

but http://localhost:28017/db/col/?filter__id=5cdf505726442b3f4114851a
gives zero because it understands the filter as
query: {
_id: 5
}

simply said, if _id starts with an number, the rest is ignored and
therefore invalid.