As a holdover from Global Point in Time reads, find, aggregate, and distinct are able to change the global read timestamp of a transaction during execution, to allow picking a time based off the shards that would be targeted. This logic has been disabled until SERVER-36312 is completed, but these commands may still set the time forward, under the assumption that doing so is safe because any operations that would cause data to move would also force a stale shard version error. There's really no benefit from allowing this behavior, and it could possibly lead to obscure bugs, so it should be disabled and re-enabled as part of SERVER-36312.