Haha ! It’s actually a well known bug in all TGM games : if you put the line piece (works with any other piece, if I’m not mistaken) way over the top and clear a line, it glitches out and makes an odd line instead of clearing everything.
Yeah, there are only 3 invisible rows above the top of the field. If you manage to stack something (usually a big mode I piece) higher than that the 4th row will be cut off.
There is only one invisible row at the top of the field. If there were three then (aside from the I-piece) it'd be just fine.
Oh, so there is. For some reason I remembered being able to stack above the well in big mode as long as you didn't go too overboard.