The answer is that in a dashboard or form-search context, Splunk has optimized behavior where it will only extract the fields that it sees in the search language of the main search string.
2 problems arising out of this.
1) referring to fields in the postProcess search that are not in the main search. Splunk didnt extract transactionType in the main search so this wont work.
2) referring to fields in the SingleValue module that are not in the main search.
The UI can also send a parameter called required_field_list when the search is dispatched, and in most cases the modules are written correctly so that they know to do this. For example if you have a downstream SimpleResultsTable, with its 'fields' param set, the table module declares to the framework that it will need those fields extracted, so the framework includes them in the required_field_list. I think the implementation of SingleValue might overlook this case but i could be wrong.
easiest solution is to sneak in a fields clause into your main search, and include the '*' in that fields clause so that it doesnt actually do any filtering.
source="*stats.log" host="*prod*" | fields transactionType *