I also filed a bug before finding this post. FB9631338. We have been able to work around this by using cordova-plugin-advanced-websocket which uses socketrocket.

We are experiencing the same issue with the final iOS 15 build we installed yesterday (19A344). Will file a bug report as well. Have been able to work around the issue temporarily by switching from the built-in web socket to cordova-plugin-advanced-websocket.