Percona MySQL 5.6 databases ignoring key length at constrait creation
By default, an index key for a single-column index can be up to 767
  bytes


–MySQL Reference Manual

The size of your index is:

191 (characters) * 4 (utf8mb4 max size per character) + 4 (length) + 1 (NULL) = 769 


The calculation may not be 100% exact, I am not fully sure about the length requirements, but I will assume it is not a bug.

The reason why it may be working initially could be because you may be using utf8, not utf8mb4 on the original system (which is 3 bytes in size per character instead of 4) or you had enabled the variable innodb_large_prefix.

In any case, I would recommend you to fix it enabling that very same variable, which will allow you prefixes of up to 3072 bytes (it will be the default in newer versions).

Edit: ALTER TABLEs on a production PXC are usually done in RSU mode.