I’m not sure if this is a bug or if it’s intentional. But a “J” shaped 7-match does not generate a critical tile. It’s hard to reproduce but I’ve seen it a few times, and I think it only destroys the two 3-matches and leaves the connecting tile alone.
What I mean by a “J” shape is a “T” 5-match with two additional tiles at the bottom of the T. I.e.:
XXX
X
XXX
More descriptively if you imagine a 3 row by 4 column block, matching tiles at row 1 columns 2 3 and 4, row 2 at column 3, and row 3 at columns 1 2 and 3.
I’d wonder if the same issue would come up with an “I” shape but I haven’t encountered that yet.
EDIT: my crude drawing with x’s isn’t showing up right, but imagine a capital “i” with the bottom row shifted one to the left.