A little google brought me to the conclusion that this is a quite common problem with multi session DVDs. (kind of like it's "by design" that way)
As long as the disc isn't finalized (closed) those first sessions don't show.
I can't say that from my own experience because I haven't burned any multi session Disk in decades.
It has been said that IsoBuster
would be able to access/retrieve former sessions.
If you could retieve your pics and then finalize that disc you're on the safe side