You are correct, there is a problem with mixing these two objects. It turns out that the MultiAnswer object uses normal answer blanks for the first answer, but special ones for the second and later answers, and the ProblemRandomize was only clearing the normal answer blanks. I have modified problemRandomize.pl to resolve the issue. You will need to get the latest HEAD version of pg/macros/problemRandomize.pl to obtain the fix.
parserMultiAnswer and problemRandomize
This forum has a limit to the number of forum postings you can make in a given time period - this is currently set at 10 posting(s) in 1 day