It seems that once a deep lizard is first aggroed, it is able to see through stealth regardless of whether or not line of sight has been broken by physical objects, transition, or time.
For example, if I am stealthed and I walk by a newly spawned deep lizard, the deep lizard does not do anything presuming I am not heard or spotted.
If I let a deep lizard see me so that it begins to chase after me but then transition out of the area and return to it many minutes later while stealthed, the deep lizard will always start chasing after me even from a great distance.
This also seems to be the case if somebody else (including NPCs) initially aggros the deep lizard. I have never seen an instance of anyone able to sneak past an aggroed deep lizard.
This is the case with many npcs. It has to do with their targeting a player never letting up. So they will continue to try to pursue you as if you had never broken line of sight. It will stop if you use a summon or another character to takes it attention off you.
This has nothing to do with the Deep Lizard's script, its just an effect that sometimes happens with everything in EFU. It's likely an NWN or LAG inherent bug, and you'll just have to learn to live with it. Its a very old occurrence, all the way back to EFUO.
It is something I recall happening with other creatures but the deep lizard is an example that happens all the time so I figured it's one way to narrow the search in case any DM cares to take a look into the issue.
Age-old issue I've seen across multiple NWN servers and single-player. Likely a hardcoded NWN bug.