This error is essentially the same as
MDEV-20726, except that is still happens for me. I'm running mariadb server version 10.4.13-MariaDB-1:10.4.13+maria~bionic.
Here is a very short example which reproduces the error (Careful - crashes the server and prevents it from restarting!)
At this point the server crashes and cannot restart, always crashing again with the error message "InnoDB: Failing assertion: !(prefix_len % mbmaxlen)".
Observations: The error only occurs if the index is a prefix index. It also doesn't occur if the collation is changed in the ALTER TABLE - going from utf8_bin to utf8mb4_bin triggers the bug, as does going from utf8_general_ci to utf8mb4_general_ci, but when going from utf8_general_ci to utf8mb4_bin it does not occur.