When a state tree tries to reenter a state that requires an event with payload data, on re-entry the payload data will contain the same value as the first entry, even if the event sent had payload data with different values.
See addl info URL for repro project.
Create a state tree with states:
Root
StateA
StateB
Create a struct for a state tree event with payload data of some kind.
State B should only be transitioned to & entered upon an event with the payload struct.
State B should contain a task to print the value of the payload data.
Create BP actor to house & run state tree.
On begin play, start the state tree logic and then send the event to the state tree with the payload data.
After a delay send the event again with different value in the payload data.
Note that between events state B has remained the active state.
Observe that the task to output the payload data has the same value on each call, despite being sent different values.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-369568 in the post.