We've been rewriting some of the game's core to improve performance, and the disappearing mission givers were an unfortunate side effect. This will be fixed in b39.

Until then, you'll have to either reload or walk a distance and come back to get them to reappear.

I know it's inconvenient, sorry about that.

can't do a mission without going into a casino - same thing

The explore mission is fine and doesn't affect it, it's the going into a casino that does it, upon leaving they are both gone.

happens after every mission now

I just entered James Tower and came back out. All avatars were gone even player avatars, no names showing either and not player dots showing on mini map either.
Like as if I was in an entirely different dimension lol. Refresh solves the issue.

Entered another casino and exited, no effect. Refreshed and re-entered site and they were back. Took both missions again, entered my casino and exited without doing anything and they were gone again.

they disappeared twice and i can't get into my casino

Likewise, did both missions, upgraded my casino level as I'd just enough chips, went back to collect on the slot master mission and no quest people (nor chip markers) visible in the square.