Doesn't sound like the intended behaviour to me? I'm still using 1281.
I have several screens where the entire screen is either wall or water, and you're supposed to have to dodge the tektites... but they don't appear anymore. They'll go back to showing if I use the enemy flags, I guess, but a big part of these screens was the unpredictability...
Download beta 18 and see if this problem still happens in that beta.
I didn't see much of anything in the changelog that would lend itself to such a change, since my build. I can't really download things right now, since I'm on dial-up, but maybe I'll think to do it this weekend. There was some stuff in the 800-era builds or something like that (I had previously been using 743 or something) where they switched around how enemies spawned.
Also, are Tektites supposed to be flying enemies or not? I vaguely recall them having no quarrels about landing on unwalkable combos, so I don't know why, even with "flying enemies can spawn over unwalkable combos" ckecked, they still don't appear?
Tektites are jumping enemies, which normally ignore walkability.