I think it is because you can't use those fields for Find requests, but someone else may provide a more complete answer.
Oh, Ok Keywords.. Thanks for the suggestion!
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A subsummary field is a "Subsummary when sorted by". Sorting doesn't apply to Find Mode, so when you switch to Find Mode in a List View, there is no subsummary.
As Subsummary parts work differently in Form View, what you can possibly do is have a script trigger to View As Form when you enter Find Mode, and then View As List again when entering Browse Mode.