Strange ER_UNSUPPORTED_ACTION_ON_GENERATED_COLUMN upon ALTER on versioning column

create or replace table t1 (i int, j int as (i), s timestamp(6) as row start, e timestamp(6) as row end, period for system_time(s,e)) with system versioning;

alter table t1 modify s timestamp(6) as row start;

MariaDB [test]> alter table t1 modify s timestamp(6) as row start;

ERROR 1907 (HY000): This is not yet supported for generated columns

MariaDB [test]> alter table t1 modify j int as (i);

Query OK, 0 rows affected (0.00 sec)

Records: 0  Duplicates: 0  Warnings: 0