Questions on mongodb failed after upgrade from 0.38.5 to 0.39.3

After upgrading from 0.38.5 to 0.39.3, some questions on mongodb are now returning 'No results!'. The native query for these questions have changed. For example :

Nativer query for a question in 0.38.5 that returns results :
[{"$project":{"hsi___service":"$hsi.service","mobilephone":"$mobilephone","metas___operator":"$metas.operator"}},{"$match":{"$and":[{"$or":[{"hsi___service":{"$eq":"combo soft | 50M/50M"}},{"hsi___service":{"$eq":"combo soft | 50M/5M"}}]},{"mobilephone":{"$eq":"MOBILOVER_SOFT"}}]}},{"$project":{"_id":"$_id","___group":{"metas___operator":"$metas___operator"}}},{"$group":{"_id":"$___group","count":{"$sum":1}}},{"$sort":{"_id":1}},{"$project":{"_id":false,"metas___operator":"$_id.metas___operator","count":true}},{"$sort":{"metas___operator":1}}]

Native query for the same question in 0.39.3 with 'No results!' output :
[{"$match":{"$and":[{"$or":[{"$expr":{"$eq":["$hsi.service","combo soft | 50M/50M"]}},{"$expr":{"$eq":["$hsi.service","combo soft | 50M/5M"]}}]},{"$expr":{"$eq":["$mobilephone","MOBILOVER_SOFT"]}}]}},{"$group":{"_id":{"metas":{"operator":"$metas.operator"}},"count":{"$sum":1}}},{"$sort":{"_id":1}},{"$project":{"_id":false,"metas.operator":"$_id.metas.operator","count":true}},{"$sort":{"metas.operator":1}}]

There is no error in the log of the server.

Hi @rjb
Interesting. I must admit that MongoDB queries just confuses me, so do you understand what the problem is?
I'm guessing that the problem might have been in 0.39.0, so can you test that version, so there's a narrower version gap to figure out what might have caused the problem?

Hello @flamber
The problem is indeed already present in 0.39.0, with exactly the same native query as in 0.39.3.
Unfortunately, I am not able yet to figure out what is wrong in the late mongodb query.

@rjb Looking over the query, it doesn't seem to be wrong and I have no idea what the problem could be, so I would recommend creating an issue and providing as much information as possible, so it's possible to reproduce the problem.

Thank you @flamber.

Fixed in 0.39.4. Thank you very much !