@299792458
I think Fukurou was preoccupied with something else that made him make such a simple mistake. Page 6 and 7 makes it seems that there is something else important enough that Fukurou was willing to think about other than just the cards. Now what that could be I have no idea.
I'm also curious why the name flashback came up again. The meaning behind the name Hal and that 87. Pretty sure there is some deep meaning there.
Also, can someone confirm if the sfx on page 9 is the same as chapter 445 where Baku was slamming the table in frustration? I think I remember in the comments in that part that there were mentions of using sound to communicate, and I think that blurry image of page 9 is the slamming that Baku did.
Edit: You know what, I'm an idiot. Assuming that Baku did send a signal to Hal through sound, then it makes sense that both Hal and Fukurou could both hear it. I'm thinking then what he was preoccupied with is the message Baku send.