[confirmed]Rakdos Cackler, infinite +1/+1 counters

I've noticed that there's a loophole that allows Rakdos Cackler to accumulate an arbitrary number of +1/+1 counters when casting it with both B and R mana available, which can be reproduced by the following steps:
- Attempt to cast Rakdos Cackler
- Choose 'Cancel' when the game asks you if you want to pay B or R
- Give it a +1/+1 counter
- Rinse and repeat
Savegame and screenshot attached. Thanks.
- Attempt to cast Rakdos Cackler
- Choose 'Cancel' when the game asks you if you want to pay B or R
- Give it a +1/+1 counter
- Rinse and repeat
Savegame and screenshot attached. Thanks.