I can only assume,...
I think, because the "mechacon"/cdvd-drive-cpu verifies the first disc as "Video-DVD" and maybe it doesn't handle the TOC then (...or... --> maybe it gives the handling for those discs to dvd-player - UDFIO-Part.).
I assume after you swap your discs, ESR (or ule) does refresh the "file-tree" (while mechacon still think, it is a "valid disc"/video-dvd), or it is because of how the parsing for "SYSTEM.CNF" woks.
Sorry,... Don't have an in-deep look into all. Like I said, it is only an assumption.
So,... I assume, it is because your swap-disc is a dvdv (...crazy things happend some time, back in the days, when I played with the dvd-player-upgrade... --> Reminds me to it. I didn't made such a test but it seems to match. If that is true, I might should retest the very first FMCBs and some stuff,...).
PS2 V7/DMS3 V2 (FW:2.4b7); Seagate Baracuda 200GB
PS2 V7/CC1.0 (FW:34 hacked v2 BM:2.1.6); Maxtor DiamondMAX9 PLUS 160GB
PS2 SCPH-30004R; NoMod+NoLaser
3xSony MC 8MB
MAX/Datel 16MB with Boot-CD
Custom FMCB 1.8b+ Beta-Build, my AIO 0.5, Sony&xRhino-Linux