I read tonight about a bug in Xoai a foundational library for DSpace and Dataverse which uses lxml lib for parsing.

Since the OLAC XML implementation of OAI-PMH requires the use of an XSI element it seems that the bug defined here https://github.com/DSpace/xoai/issues/67 and discussed here would apply https://github.com/gdcc/xoai/issues/141