There was a qBound check added to the QHeaderViewPrivate::setDefaultSectionSize method with the commit faff43348bfae5cfc709fabe9d2698fc3063c050.  This causes problems, that were partially described in the QTBUG-69431.  In short: if you call setDefaultSectionSize with a value, that is less than current minimumSectionSize, the size would be set to the current minimum.  That is not much of a problem, as you can fix that by proper sequence of setting minimum and default sizes.

Now the real problem arises, when you try to configure a table in ui file, by setting minimum and default sizes to one of its headers.  The generated ui_*.h file will put setDefaultSectionSize call before setMinimumSectionSize, and the former request doesn't set the desired value.  As a result, generated table does not reflect its settings.

Attached project recreates this issue.