It's a bit odd, but I found splitting the values apart and putting them back together works:
eval names='ExtendedProperties{}.Name', vals='ExtendedProperties{}.Value' | eval temp=mvzip(names,vals) | mvexpand temp | rex field=temp "(?<name>[^,]+),(?<val>[^,]+)" | eval {name}=val | transaction Id
updated to add back the deleted terms in angle brackets. - dmj
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