Thought WIRIS filter had editor dependencies ... those being the atto and tinymce editor plugins as well.
https://moodle.org/plugins/browse.php?list=set&id=66
"Examining the XML file" ... which xml file? The moodle_backup.xml is the roadmap for the entire course. There are other xml files.
Those that had blank values ... mind sharing what xml file and what were the tags around those blank values? You said they were "activity with a title" ... uhhhh, thought activity title was a required parameter. Wouldn't hurt to put a single character (oh, say a 'x') in those that a blank or containing 2 CR's. Re-package the .mbz again such that moodle_backup.xml is at the root of the .mbz file. Then try it again!
"$a contents: 800" is a clue ... but where ... maybe in one of those xml files an ID number 800? Those activities with no title that had 2 cr's ... was ID number or other designator 800?
'SoS', Ken