I believe that the issue could be related to the fact that Shopify has silently transalted the app’s listing Plans to Spanish. A bit worried that it has been done silently given that so far the display name was the only way to identify a plan.
We also found that it seems that Shopify has released a plan_handle a few release ago: New planHandle field for managed pricing app subscription plans - Shopify developer changelog
A solution explaining how to use the handle be found here: Managed pricing: planHandle is null