The Stone Brain Doesn't Work in Most Cases
While the bug has been reported and experienced quite a few times before, it's been a while and I wanted to submit my own account with a log and screenshot. Long story short, I really, really want to Stone-Brain-Drain my opponents in Standard by duplicating TSB each upkeep with Mirrorhall Mimic (and Mirror Box). It would bring me such joy, but alas.
An example of The Stone Brain working properly:
1: Start match with Sparky
2: Cast The Stone Brain to the battlefield as soon as you have two untapped lands
3: Next turn after upkeep, activate The Stone Brain. It DOES work as expected! The menu shows up and you can exile Sparky's Affectionate Indriks.
An example of how The Stone Brain glitches (the only steps listed are those taken to create the legendary copy of TSB which is what is activated at the end):
1: Start match with Sparky
2: Cast The Stone Brain to the battlefield
3: Cast Mirror Box to the battlefield
4: Make The Stone Brain into an artifact creature (I've been using Tezzeret, Betrayer of Flesh)
5: Enchant The Stone Brain with Ghastly Mimicry
6: Wait until next upkeep when a legendary token copy of The Stone Brain is created and then activate it for free due to Tezzeret's passive ability.
7: Activate the legendary token copy of TSB
8: No menu pops up! The Stone Brain does NOT work as expected
Hopefully this helps identify the issue. I'm attaching log files from a game I played with Sparky that follows the second set of steps listed, as well as a screenshot of Step 8.
Thanks for taking the time to read. Here's to hoping that this can be resolved soon!