I have just noticed that casper-rw persistent files are not working with Ubuntu 20.04 when booted from an ISO file.
Persistent partitions are again working, however persistent partitions do not work with "persistent-path" and only one persistent partition is allowed per USB drive.
This means that a multiboot, multipersistence USB drive is no longer possible using ISO files.
I have also just tested BIOS and UEFI boot modes of a Persistent UNetbootin USB drive and persistence does not seem to be working with Syslinux either.
Hopefully someone will point out where I am making an error, or show how to fix this problem.