This seems to be a bug rather than feature request. On Windows it works exactly as expected.
On iOS you can search on custom fields within custom types but not on custom fields within the preset types.
(Admittedly very limited testing and I only have Windows and iOS to compare)
OK - should have read more carefully
Thanks for the feature request and thank you Andy for the bug report. I'll test this issue out later today and document it as a bug/issue for fixing as soon as possible.