AntennaPod just fixed this 20 hours ago and has multiple known issues that are likely leading to this rather than it being the fault of secure spawning:
https://github.com/AntennaPod/AntennaPod/pull/7751
Please don't tell people to disable secure app spawning because of an app bug.
If you look at those issues on their tracker, only a subset of the people experiencing them are GrapheneOS users. GrapheneOS users are much more likely to report issues to app developers.
