This following query doesn't work:
select from Chat let users = in('HasChat'), brain = out('CreatedFromAssistant') where #22:1 IN $brain AND ( contextVariables['local:fin'] = '7:106514432' OR @rid = '#7:106514432')

If I remove the OR condition inside the parhentesis works.

My first thought is to debug the execution and at a certain point the query is "flatten". Not sure why, maybe for index optimization? In the class SelectExecutionPlanner, method optimizeQuery() starts the computation of the flatten.