Thanks to @pianoyeg94, it turned out this led to incorrect definition of recursive cardinality update functions for such tree config.

We have the following code in the bps_tree.h:

#elif defined(BPS_INNER_CARD) && !defined(BPS_INNER_CHILD_CARD)

The expression should check BPS_INNER_CHILD_CARDS instead of BPS_INNER_CHILD_CARD.

I'd suggest to either fix and cover it with tests or remove this config completely as not used by Tarantool.