Type conversion error on ALTER TABLE on Galera cluster in TOI mode

When doing a schema change in rolling schema upgrade (RSU) mode on a galera cluster, changing the type or length of a column, it is known that errors like:

[ERROR] Slave SQL: Column 6 of table '...' cannot be converted from type 'varchar(1024 octets)' to type 'varchar(18600 octets) character set utf8', Internal MariaDB error code: 1677

when changing data while only part of the nodes have been upgraded yet, unless slave_type_conversions=ALL_LOSSY,ALL_NON_LOSSY is set.

Now we seem to have had a case where the above error occured on a cluster in TOI mode.

The ALTER TABLE was using ALGORITHM=INSTANT to change the length of a VARCHAR column, and wsrep_slave_threads was set to 8, so this might be a parallel execution race condition?