Is that bad? I think the new result is ok. The date part of TIME'10:10:10' is DATE'0000-00-00'. You can see that explicitly when casting to datetime, it'll be DATETIME'0000-00-00 10:10:10'.

There's no logical explanation why the date part of TIME'10:10:10' shoud be '2010-10-10'. And I don't consider "because we internally cast time to a string and then to date" a particularly convincing explanation.

Compare, for example, with double→integer cast:

MariaDB [test]> select concat(1e15), cast(1e15 as unsigned);

+--------------+------------------------+

| concat(1e15) | cast(1e15 as unsigned) |

+--------------+------------------------+

| 1e15         |       1000000000000000 |

+--------------+------------------------+

when casted to a string, it's "1e15". If you cast this string to an integer, you get "1". But double→integer cast is not using strings, that's you get the correct result — the integer part of the floating-point number.

time->datetime is now works according to the standard, it uses CURRENT_DATE. So,
CAST(TIME'10:10:10' AS DATE) is CURRENT_DATE now.