Currently, if you have a subtask that meets a filter requirement but the parent task it is under does not, that subtask does not show in a view when subtasks is set to "expand all". You must set it to "as separate tasks".
However, showing ALL subtasks as separate tasks can make a list very cluttered and overwhelming. That is where "expand all" helps, as it keeps things more tidy.
I think that if a subtask meets a filter requirement but the parent does not, expand all should include that subtask. This would mean other tasks are still neat, but you still see subtasks which need working on.
There could also be an argument to include these particular subtasks in "compact" mode.
With the current options, subtasks which meet filter requirements but where their parent does not are easy to forget about.
I think the best option would be to have a toggle in the "subtasks" dropdown for this.