I have found that there definitely seems to be a problem with the option to split tags, but I have not managed to recreate the symptoms you describe so I am not sure if what I am seeing in my testing is the same issue or a different one.
It is quite likely that I do not have test data set up correctly to exactly reproduce your problem. It would be useful for my investigation if I can get hold of a copy of the metadata.db file from your calibre library, and also the calibre2opds profile.xml you are using so I can use the same settings. If you are willing to provide this I would suggest that you either attach this (zip it to reduce size) as a comment to the issue
#c20-231 issue or email it to calibre2opds at gmail dot com.