"Extra" snippets added in UI to a picture def are not saved

I followed the tweak in support.bible issue 12786 to bottom-align pic captions to the bottom page margin. It worked great and I got an error-free print run. Saved a separate copy for archive purposes, and reopened this publication the next day in PTXP. To my dismay 15 pages were suddenly underfilled!

I finally tracked it down to two span-width pics where I'd used the "one-off tweak" because the style I'd created missed span,bottom pics ('\loc:bX|fix' catches bl, br, bo, bi but not "b"). Two of these span,b pics. (I'm sure this style name can be modified to include "b", but I didn't have time to experiment.)

So anyway, the first pic below shows the correct caption alignment, with the "Extra" code tweak to bottom-align this single caption. The second pic shows the same project, reprinted after closing/restarting PTX Print: the three pics with this tweak Extra box entries had lost the contents of the Extra box.

I repeated this several times so I'm sure about it. The tweak works great, but PTX Print doesn't seem to save my Extra entries when shutting down. I made a couple attempts to add the Extra bit directly in the piclist file but didn't succeed.

Reproduce the issue in PTX Print by adding an Extra snippet to a span,b picture definition, saving, closing, and reopening the publication. Trace how picture-definition Extra entries are persisted and restored; done means the entries remain present and the printed captions retain their alignment after restart.