My guess is this is a bug on parse-server and not Parse-Swift and someone on the server team should probably pick up the Q&A here…
I say this because it doesn’t appear when using explain in Postgres. When I added explain to the Postgres adapter on the parse-server (PR 6506), I made sure it’s always returned in JSON format so it can always be decoded:
Looking at the explain results you’ve shown along with some of my own testing, the Mongo version of explain isn’t always returning a JSON version of explain. CloudCode and the JS SDK most likely show results fine in because JS isn’t a strongly typed language and isn’t trying to decode JSON to specific types. Conversely, Swift is, and expects everything from the server to be in a “correct” JSON format or else it throws errors. The mongo version of explain doesn’t appear to be in a correct JSON format.
Update: the problem is a mongo explain isn’t being returned within a JSON array.
If what I said above is true (the server team will need to confirm), there will most likely need to be a fix in this file parse-server/MongoStorageAdapter.js at master · parse-community/parse-server · GitHub. I found some info about explain in mongo (but I don’t use it, so I won’t be able to provide any additional insight beyond this): explain — MongoDB Manual
A workaround could be to use your Cloud Code explain function and only return the fields your care about to Parse-Swift.