A source or not a source

When I use the timestretch function, sometimes the results appear in the Source list. Then they disappear from it when reopening the session.

See the 2 screenshots attached: I’ve been playing around with my sound ‘woosh1’, and some of the stretched versions appear in the source list, but others don’t. In the first image the source list includes 2 of the 5 stretches I made (woosh1.18@20-494 & @30-258) but saving, closing and reopening the session they have disappeared from the list (though they are in the timeline and in the Region list).
This seems odd.
(all of the stretches were done with the same varispeed resample option)

Screenshot 2020-11-16 at 12.11.41 Screenshot 2020-11-16 at 12.12.11