People haaaaated not getting talismans of any kind, and actually asked to be able to more easily get lower-tier talismans in the higher floors, so we acquiesced accordingly (and apparently it's
still not enough). I think there's more support in them being there if not (and if you don't want 'em, just sell them on the spot from the loot popup for some extra Gold). We agree that we don't want to flood you with lower-floor talismans, though, so it's a difficult balance to strike, and thus they're still relatively rare.
They're not making the rarer ones less likely, though. The drop rate of the main talismans for a given floor is explicitly controlled to produce a chance that will meet certain targets for "average amount of runs per set", and the probability of other kinds of rewards are judged to ensure those rare talismans drop at the intended frequency.
No one has the (intended) ability to place negative effects through Immunity. However, we've had problems in the past where certain methods of applying negative effects got around Immunity (for example, Grievous Burns able to Stun the unstunnable Sentinel Harpy). If such a bug exists, though, we need to know more about what characters/effects were being used to investigate it further.
There's also the matter of getting Immunity could appear to happen at the same time as gaining a negative effect (end of one turn applying something while start of the next turn applying something else), which can make it look like negative effects are being applied to an Immune character, but it's just the game isn't visually showing those separate steps of combat resolution as separate. While the visual timing of something is something we want to improve, in this case combat is functioning as intended (since Immunity is gained after gaining a negative effect, and gaining Immunity doesn't remove negative effects).