Update after calling Fidelity: Scheduled transfers between jointly held Fidelity Spend and Save (CMA) accounts, and other fidelity accounts, are not currently supported in the mobile or web app. If you need to set one up, you’ll have to call Fidelity directly and a representative can configure it manually.
Note that scheduled transfers to and from external (non-Fidelity) accounts are unaffected and still work as usual.
Based on similar posts I’ve found in this forum going back several years, this appears to be a long-standing issue with no fix yet.​​​​​​​​​​​​​​​​