Backup, with “Generate ISO” checked is not working correctly.
My first attempt produced a correctly labeled zero byte ISO file.
Subsequent attempts, again selecting the "Generate ISO option, produce a correctly named and sized .ipf file.
I last successfully created an ISO file in Core182 stable.
The ISO backup has a first step of downloading the iso for the version that you have installed.
It looks to get this from the download site but that site only has the iso’s for the Stable Releases so currently the latest version is 182 but your Testing release will be 183 so it will fail to get the iso which will end up with 0 bytes.
Hence the ISO backup can not be tested in the Testing Release.