jmo moonphase
I think since there are multiple reports that implicate a particular code path it might make sense to file an issue on the GrapheneOS issue tracker. Please include as many details as possible (GrapheneOS build number, device model, USB-C adaptor/headphone make and model). Include that unplugging the device before the call ends avoids the reboot.
Since there are two of you, it would be good if one would post the issue to the tracker and the other would add a comment. But if either/both don't have GitHub accounts, please post a report in this thread (all descriptive matter in one post) and either @other8026 or I can make the GitHub post and refer to the two posts in this thread.
One last question... does it matter whether the call is ended by you or by the other party? Actually I guess there are four cases: you call out and you end the call; you call out and the called party ends the call; you receive the call and you end the call; you receive the call and the calling party ends the call. I sort of expect that all four cases will cause the same crash, but if only some of them cause the crash that might be useful information for a developer looking into the issue.