Aggregation slow without hint, but winning plan same as with hint

I have a very slow aggregation (240 sec), but an index exists, which should match the query perfectly.
If I add a hint to the aggregation with the index name it is blazing fast (0.2 sec) on 120M records.

Now, if I compare the explain output between the aggregation with and without hint, they look the same.
Both use a IXSCAN on the same index.

What am I missing?

MongoDB 5.0.9


{ $match: { 'action':'signed',
            'quality':{ $in: [ 'QES', 'AES', 'SES' ] },
            'timestamp':{'$gte': ISODate("2023-01-01T00:00:00Z"), '$lt': ISODate("2023-02-01T00:00:00Z")},
{ $project: { _id: 0, 'quality':1}},
{ $group : { _id: '$quality', "count":{$sum:1}}}


    "action" : 1,
    "quality" : 1,
    "timestamp" : 1,
    "paid_by" : 1,
    "user.business_id" : 1,
    "user.username" : 1,
    "legislation" : 1,
    "tsp_type" : 1
       partialFilterExpression: { 
           'action': 'signed'

If your usual use-case is the query shared where you have equality match on paid_by and range match on timestamp, your index would be better with paid_by before timestamp.

I don’t think your $project helps but it may negatively impact.