So, where we're at is that it works on all files newer than 2012 and spotty on older files?
The process of saving to custom list is simply to copy out of one database and copy back into another database. What you're reporting seems to be a database issue. I won't be able to test it again till Monday but, I'll try some older posts. With your storage age set to 2000 it should just work because the databases shouldn't be getting purged.
the number went from 120 to 2 entries and when I opened the list in a tab (double clicking on the list) it was empty. By now I have deleted the list, added a new one. But right at the moment it's not working at all. I'm going to keep trying with different groups though.
How about doing "Show All" when you view this list? I'm pretty sure "Display Age" still applies as do any other filters you might have set. You seem to be reporting that they're spontaneously vanishing which is typically a filter issue(that they're still there but, hidden). That the age or old/new status of the posts is simply hiding them or one of the filter profiles.