Why have “unidirectional ‘mapping’” tables?
I agree that it is a strange table.

No uniqueness constraint, so dups can occur.  I would change INDEX to PRIMARY KEY.

No index the opposite direction, so unlikely to use it that way.  I would add another column and INDEX(bin_id, board_id, pin_to_board sequence)

On the other hand, perhaps there is a lot of activity in the table -- such as updating all the sequence values frequently.  Still, InnoDB would really prefer to have a PRIMARY KEY, and that triple seems like the 'right' PK.

I would probably work harder to avoid potential timestamp dups.