It depends what intermediate you pin, but the CA can also choose to change the root certificate they use at any time like Let's Encrypt did in 2024 when the CA that signed their cross signed certificate stood to expire. Plus, depending on where you get your certificates from, the reseller certificate may already be an intermediate rather than its own root.

You should probably pin the highest certificate in the chain that's going to stay current for as long as possible. Or, if the goal is just "I don't want people snooping around in my app's traffic" rather than "I want to protect against a rogue CA being used to hijack my customers' traffic", reuse the private key in the CSR and pin that, it'll get the job done.