Stripe Plugin Inconsistently Sending Plan Name in POST Data

Hi, I’m having an issue where about half of my Stripe subscriptions are returning as 400/errors, the reason being “parameter_invalid_empty - subscription_data[items][0][plan]”.
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But in my Stripe logs, there are many, many instances of the plan being sent through properly, so I know that - the way I have it set up - it is working at least half of the time. I’m using the default Stripe plugin, I believe, with the action “Subscribe the user to a plan”.

Do any of you know why this action might sometimes send the plan name, sometimes not? All of the options possible in the dropdown (from an option set), I have verified they each have a defined plan ID. Is there something I’m missing? Is there a stability issue? What could cause the POST data to be inconsistent?