When any of the inputs to a fast path node are connected from a struct member from multiple splits or splits combined with breaks, all input connections to the node will be effectively ignored, instead using their defaults. Note that if any of these pins are left unconnected and their values can be edited directly on the node, this will correctly be used as the defaulted value.
If the node uses the slow path for any reason this issue does not occur.
Create a struct containing a float.
Create another struct containing the previous struct.
In an animation BP, add the outer struct as a variable and give the nested float a value.
Use the float in an animation node by breaking both the outer and inner structs. Notice how the fast path is disabled and the input value is correct, by hovering over the input pin.
Now use the float by splitting one or both of the structs instead. Notice how the fast path is enabled, but the value is incorrectly defaulted.
Expected: Values should be picked up correctly even when splitting nested structs.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-71264 in the post.