Update was skipped (need_update was false) because compare_record()
used HA_PARTIAL_COLUMN_READ branch and it skipped row_start check
has_explicit_value() was false. When we set bit for row_start in
has_value_set the row is updated with new row_start value.

The bug was caused by combination of MDEV-23446 and 3789692d. The
latter one says:

  ... But generated columns that are written to the table are always
  deterministic and cannot change unless normal non-generated columns
  were changed. ...

Since MDEV-23446 generated row_start can change while non-generated
columns are not changed.

Explicit value flag came from HAS_EXPLICIT_DEFAULT which was used to
distinguish default-generated value from user-supplied one.