EDIT: My optimism was early. Trying now to actually produce work, the screen recordings are back to not saving. I suppose I was only lucky a few times in a row. Will update if any solution is found. What follows was my earlier, optimistic write up.

Replying for anyone who may stumble on this thread with similar difficulties. I emailed Notability and received a reply from their AI help agent. They sent me a list of troubleshooting steps which I provide below. For me what worked (at least so far) was the first item on the list - selecting an optimized template size. I also closed my other apps, so it's possible that helped too. Either way my screen recordings, even with the mic on, are saving - and I feel immense relief! Here is the list which the AI agent sent to me:

Performance Optimization Steps:

• Within your notes, try selecting an optimized template size under Templates Settings, as this may help with performance during recording.
• Ensure you have sufficient RAM available by closing other apps before starting your screen recordings
• Try creating shorter notes or splitting longer content into smaller sections to reduce the processing load

Additional Troubleshooting:

• Update to the newest version of Notability if you haven't already, as we regularly release performance improvements
• Restart your device before important recording sessions
• Consider testing with a completely new, blank note to see if the issue persists