what is MDL? (e.g. 'MDL semantics' or 'MDL locks') Probably Galera related
It stands for "MetaData Locking" seeing the sources of MySQL https://dev.mysql.com/doc/dev/mysql-server/8.0.0/mdl_8h.html

It's a way to keep an ACID database by avoiding concurrent access to the same data. The idea is that the metadata of a table are "locked" when a query inside a transaction uses this table. Doing so, another transaction will not be allowed to do DDL queries on these locked tables.

For instance, if you do a START TRANSACTION then a SELECT * FROM t in one client application, then the metadata of the table t are locked. No other client will be able to do a ALTER TABLE t ... because it would mess with the table structure while the 1st client transaction is still opened.

More infos in MySQL doc https://dev.mysql.com/doc/refman/5.5/en/metadata-locking.html