Platform: steam, Win10
When the Scorpion’s hide in sand changes a big gem into yellow, it will still show as a big gem. However, if it is converted to purple by night 1 right after, it becomes a normal purple gem. Also, if the big yellow gem moves down in the same turn because the gems below it get removed, it becomes a normal yellow gem.
I see this a lot because I usually turn almost the whole board into purple and the last few gems will be big gems. After that, when I cast an attack spell, the same big gem gets converted to yellow (big) and then purple (normal). Happens every time, very consistent.