I installed CapCut on the iOS 18 Seed 1 release. I performed the steps that you described, but I was unable to reproduce the problem with audio dropping out unfortunately. Do you think you could file a bug report for the issue?
https://developer.apple.com/bug-reporting/
What would really be helpful is to have a sysdiagnose when the issue occurs with a note from you giving the timestamp when the issue occurred; the feedback app mentioned on that page should (presumably) make it easy to go that.
Thank you for your feedback.