I wanted to update this post with, what I believe, was the cause of the issue. Before I do, I have decided to offer my users a free trial without the need for checkout, this way I don’t need to use Stripe for a free trial and can simply ask the user to upgrade to a paid plan when trial expires.

One issue was when creating a product, I was trying to apply trial directly to that product. That is actually legacy setting. Setting a default trial period per price is no longer recommended in Stripe and is incompatible with Checkout. Instead you can set up trial per subscription
https://stripe.com/docs/billing/subscriptions/trials

Also, if you bill monthly you can’t have add a free trial for less than a month, so “Current date/time + (months):1” wouldn’t work. I may be wrong on this as I didn’t investigate much but for me it only worked when my trial length was one month, matching the billing cycle.

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