I believe that fixing this requires an undo log format change.
MDEV-17197-WIP-10.3-7e869a2767c849d266ffbf1def9b93ffae7cd428.patch modifies trx_undo_page_report_modify() so that no data field contents is written for adding a column to the metadata record. This would cause an assertion failure during rollback, in the function btr_cur_trim(). Rollback depends on the presence of the default value in the undo log record. The function btr_cur_trim() would remove trailing fields from the metadata record during rollback.

It seems that instead of copying the default values to the undo log record or omitting the values altogether, we should write something special that allows the equivalent of btr_cur_trim() to be performed during rollback.

Because long default values should be rare in practice, and because this bug does not cause any corruption, I do not think that it is feasible to fix this in the stable 10.3 series. The bug can be worked around by not specifying long DEFAULT values for instantly added columns, or by adding the , FORCE keyword, so that the table will be rebuilt instead of performing instant ADD COLUMN.