In Gen 9 Nat Dex, changing anything in the Details tab of the teambuilder causes the Details preview to suddenly display the Pokemon's Hidden Power type.
Note that clicking to another Pokemon and back will reset this, and changing the Details back to their default state will make the Hidden Power preview to return.
This happens even if the change you made was setting the Hidden Power type, in which case the oddity is not the sudden appearance of the Hidden Power preview, but the sudden disappearance when clicking away and back.
Something similar happens with Dynamax level in Gen 8 or Gen 8 Nat Dex.
However, unlike with Hidden Power type in Gen 9 Nat Dex, changing Dynamax level will actually persist in the preview if you click away and back, unless the Dynamax level is changed back to the default 10.