pqueixalos reported this to me privately so I am creating this issue on his behalf.
We found an issue in versions 4.10.+ (4.10.2 and 4.10.3 for sure).
When processing a two phase distributed query with an explicit fl parameter, the two phases are well processed, but the GET_TOP_IDS retrieves the matching documents fields, even if a GET_FIELDS shard request is getting executed just after.
=> id is retrieved during GET_TOP_IDS phase that's ok:: it's our uniqueKeyField
=> title is also retrieved during GET_TOP_IDS phase, that's not ok.
I'm able to reproduce this. This is pretty bad performance bug that was introduced in
SOLR-5768 or it's subsequent related issues. I plan to fix this bug and add substantial tests to assert such things.