[Valiant]

Well I'm glad my ramblings helped you narrow down the source of the issue, now as for a fix, this might not be so hard.

My only guess is that perhaps the textures in question are not stored in the map/textures folder. As such, with the default paths (the ones used by RTCW for its own original textures) and identical names for those textures - they may serve as overwrites for the originals. This would happen much in the same way that a server configuration script for a bot waypoint may mess up someone's original server.cfg settings if its overdone or the same way a custom pk3 can overwrite a default model or skin in any Quake3 Engine based game.

To to summarize - their textures "should" be in a "mapname/texture/customtexturename.extension" type path but instead are using a default type path, and using an identical name to an original texture.
So you load the game, and it thinks it needs to look in mp_island.pk3 for the source image "main/textures/castle_wood/cwood_mo5cb.tga" (seeking the overwrite and following the path specified) but since that file does not exist in mp_island.pk3, it displays a missing texture instead.

It may sound crazy but what I would do is make a backup of mp_interception, your culprit file then see if that second path/filename I've highlighted in red is in fact in the pk3. If it is, simply delete it - and that map may just retain the path thereby reading the original RTCW wood texture, instead of its modified one in the overwritten (duplicated) default path. If this works it should cease the conflict with mp_island and other maps that use that wood texture. If not, you can restore your copy of interception from the backup and probably just contact the author via E-mail. If he/she isn't already aware of it, it's possible the map may need to have the original wood-texture's reapplied over his modified ones in radiant, or simply having his altered textures renamed and moved to a valid self-contained path for the map - then reapplied/compiled.

P.s. - Yes, it does read all of them into memory at once upon loading of the game, notice the wolfMP console window displays a list of your pk3's as it fires up? try deleting one of the pk3's while the game is running and it'll say its in use even if its not currently loaded