It's not a bug, you need to have a team that can either kill that mob before the unkillable or strip it off with geo. 

Yes Harbby, however when that buff goes up, geo can use his A2 to remove it. how is that intented for geo to not use his a2 for 34 minutes ?

Creeping Petrify - 4 Turn Cooldown  Removes all buffs from a target enemy, then attacks them. Steals all
buffs if the target is under a HP Burn debuff placed by this
champion. Reduces the cooldown of Quicksand Graspby 2 turns if the
target is killed by this skill while under a HP Burn debuff placed by
this champion. 

Does the strip land, i.e. what is your accuracy.

Kill the champ landing the uk first.

Fight on manual and don't use the tm control, i.e. let the enemy take it's turn to use up the uk.

You understood him wrong. there is no problem with accuracy or the stripping . The problem is the AI which does not use the A2 during auto-battle.

I had similar problem with spider where geo did not want to use his A3 on spider queen and used only his A1 (only if i focused on the queen then it worked how it should)

Thanks for the clarification.

Any AI issues you can fix with presets. Make his A3 a higher priority attack.

That being said, I sent my geomancer in to test without presets and his A3 was his first attack. He used it regularly for the rest of dragon 19 as well. 

He will use it on a regular until the buff appaears on the mob if there is a mob with that damage immune buff on he will not use his A2 on it, as he thinks to him self it will not deal damage then it should not be used. if there is more than one mob and the other mobs don't have the damage immune buff, he will then use his A2 on the other mob and ignore the mob with the damage immune. having dragon 24 on farm i can guarntee you this happenes there as well though luckily sometimes it takes less longer for the 3% rng to proc several times in a row for it to resist the effects of Turn meter reduction. which only leads to 1/10 games lasting 18 mins 

The real reason for this is because when the game thinks that it doesn't need to use a skill to finish off the target, it won't. This is especially problematic in situations like when Dragon is brought down to almost death. The difference between 3% and 0% on Dragon can be pretty high, so if you're running a solo clear champ like BEK, you have to explicitly tell it to prioritize A2, or else it'll just sit there and use autoattacks instead.

For your specific problem - if you put the A2 on priority, it'll use it even if that unkillable buff is active.

Nono i'm 100% aware of that, only reason i want Geo to always use A3 whenever it's on CD was for when i was weak geared, because my comp would die if someone would get a turn that wasn't stunned or cc'd but yeah just hoped his AI would be improved for him to understand that his A2 can remove immune damage buff and that he can use it even if the target was not going to take damage ...

I don't understand your problem. If you set up your team AI, he will use his A2. He will even do so, if you set his A3 as first and his A2 as second priority.