Hello,
Each new release of phpdocx adds new features. In the case of MultiMerge, a lot of improvements (options, in-memory merging) has been added to the class since phpdocx 5.5. That PHP notices were removed as well.
If you need a patch for phpdocx 5.5 to remove the PHP notices please reply the topic again and the dev team will post it.
Regards.