Have you found any solution for this? I have noticed that it's now possible to have both a one-time price as well as a recurring price, but the behaviour of Odoo is not exactly what we want:
As soon as a product with a recurring price is added to a quotation, Odoo uses the recurring price - even if the sale order has no subscription template assigned to it.

v17 is still the same problem as v16 unfortunately.
I had a meeting with the product manager from subscriptions I think around May-June and he basically couldn't care less about the problem.
v17 would be "maybe" we look into it but as it turned out, nothing changed.
And i also don't believe anything will change anymore, they are just selective deaf to this problem they caused.

I have started development on a custom module to solve some of the issues with mixing different recurrency in a single quotation.
I'm moving the recurrency field to the sale order line so I can change it per order line.
If the order is confirmed, I split off lines with the same recurrency so monthly becomes an SO, quarterly becomes an SO and yearly becomes an SO and so further.
This way I have better control over this and I get back 1 single quotation for customers to sign and pay instead of this madness they created.

Probably we could also do something to handle this scenario you are explaining.
If recurrency is empty (not set) on sale order line, then we treat the line as one-time purchase instead of recurring.

So far I already got a the majority working, but I still have a lot testing to do as the entire quotation template stuff is also hanging related with this whole mess they created.

@Fabio: Many thanks for your time and the detailed reply. The response from Odoo does not really sound promising at all, but it's great to hear about your progress.
I would be open to help with the development and/or testing, shall we discuss this via another channel in detail?