How to insert '1001-01-01 00:00:00' into MariaDB when altering a table?
The alter is failing because the row has a value of null for the TS column. If you remove the 'NOT NULL` from your alter statement it will work correctly though you will still end up with the row you first inserted with a value of null for TS.
You will get the same error if you do the alter without trying to set a default. The alter you are trying to use will not replace the null value and, as a result, you are getting the error.
You may change the value of the row before the alter but, as you have it written, the alter will not work as you've seen.