Mariabackup is incompatible with the MySQL 5.6.36 change
Bug #25551311 BACKPORT BUG #23517560 REMOVE SPACE_ID RESTRICTION FOR UNDO TABLESPACES
that was merged to MariaDB 10.0.31, 10.1.24, 10.2.7, 10.3.1.
With this change, it is possible to let InnoDB create undo tablespaces in a previously created instance. In this way, the undo tablespace IDs will not necessarily start from 1.
To fix this, Mariabackup should initialize the variable srv_undo_space_id_start in both backup and prepare, and it should use the value in all formulas. For example, the macro definition of fil_is_user_tablespace_id() is incorrect.
Perhaps Mariabackup should read the first page of the undo001 file and determine the first undo tablespace ID from there.