I went into the Five Flagons to visit Lynn for the first time. After meeting her, I went downstairs to hear the play. But no Raelis, no ticket seller, nothing.. just Samuel wondering where they went. So I reloaded from after seeing Lynn and went to the Sea's Bounty to finish the first quest. After doing that (and a few other things, like Jaheira getting cursed, running into some Shadow Thieves and bandits), I returned to the Five Flagons, visited Lynn, and got the next quest. When I went back downstairs, Keto is gone and then when I went back down to the playhouse, Raelis is there, and the play cutscene fires as usual. So is this a bug with Assassination?
Note that I also have the Multiple Strongholds (Baldurdash) tweak from the G3 TweakPack, Keto, and several other mods installed.