I have a similar issue with my Pixel 8, GrapheneOS, Cheogram 2.17.2-2+free from F-Droid on 2024-11-07, and Pixel Buds Pro (OG). When calling from the bundled phone dialer, I randomly get different behavior where the Bluetooth headphones sometimes don't work, or there may be rapid flipping between headphones and speaker.
I experimented by making a call directly from Cheogram, and the call seemed to start making noises from the phone speakers, then the call ended up on headphones later. So if I'm right about what happened, it could be a Cheogram bug. I reported the issue in their support channel.
If others have this issue with the same or a different configuration, it would be good to know.