Did you figure this out? Same thing is happening here.

Did you ever figure this out?

Did you ever find a solution to this? I'm having the same issue

Correct me if I'm wrong, but is_trial_period does not exist when verifying receipts locally, that is a JSON field returned from apple's rececipt verification backend. For renewable subscriptions the only field available is 1719 "subscriptionIsInIntroOfferPeriod" - which always shows as 0 for FREE trials