I see that it can be a bug, as a workaround execute the first query, if you got a error, try the second/third that should do the trick, while reporting it as a bug to mariadb jira tracking / mysql error trackingReading docs i didn't found error too
v5.7
| ALTER [COLUMN] col_name {
SET DEFAULT {literal | (expr)}
| DROP DEFAULT
}
v8.0
| ALTER [COLUMN] col_name {
SET DEFAULT {literal | (expr)}
| SET {VISIBLE | INVISIBLE}
| DROP DEFAULT
}
v10.5
| ALTER [COLUMN] col_name SET DEFAULT literal | (expression)