The log uses the correct sourcetype, however I cannot get the line merging to work as expected. Each line is a separate entry in the search output and it looks like the SHOULD_LINEMERGE is just ignored, along with MUST_BREAK_AFTER.
This doesn't really fix my issue. I've got any number of different key-value pairs before the last line. They can be in any order and there isn't any known initial entry. The only known line is the last one.