Its possible that one or more sessions became unreadable and may have appeared to have "disappeared", but just because they aren't visible to the recorder anymore doesn't mean that they no longer exist (of course a lot of good that does you).
Its also very possible that these sessions "disappeared" at the same time that you added new sessions, and that there is no real connection between the two, only coincidence. What is NOT possible is "recording over" existing sessions with new ones, like one might with videotape. It's a "write once" medium, and therefore by definition, you can't overwrite older sessions.
I've never seen this discussed, and I could be very, very wrong about this, but it seems to me that "erasing" a session from a DVDR may not really erase it, but it may just remove any pointers to it or zero out cataloging info sort of like on a hard drive. It would make sense, after all, since you really can't overwrite. If that's true, there should be ways to recover "erased" sessions though software on a PC or Mac.
There is a limit to the times you can write new sessions to a disk, and finalization will eventually be needed. Otherwise, there comes a point where the startup routines, which include a short test burn, can no longer occur. In fact, if you leave a blank disk in your DVDR and turn the power off and back on 100 times without finalizing, it will probably reach that point and become unuseable just from going through that routine 100 times.
There's no place like 127.0.0.1