After setting maxEntityCramming to 1, every entity riding another (or being ridden) get suffocated by this gamerule. As Jeb_ mentioned: "In other words, if one entity tries to push more than 24 other entities".
Passengers do not push ridden entities, and ridden entities do not push passengers, so maxEntityCramming should not count ridden and passenger as crammed ones.
Now passengers suffocate, but ridden entity doesn't when maxEntityCramming is 1. Both shouldn't suffocate.
Also 2 passengers of one boat shouldn't count as crammed entities, because they don't push each other. Now are counted as crammed. To reproduce set gamerule to 2, push a pig into boat and get into the boat. - this was fixed
All entities that are merged with relation passenger-being ridden should be counted as one combined entity, even when creating a tree-like relation (pig riding pig riding cow riding chicken riding cat riding dog), because they don't push each other.