It seems that with 4.20 users are able to change a dispatcher's type from Multicast Delegate to something else (float, int, etc.) Doing so will cause any node that references the dispatcher via call, binding, etc. to error out and cause the blueprint to fail to compile. It is also not possible to change the variable type back to Multicast Delegate.
This appears to be a regression as the dispatcher's variable type cannot be changed in 4.19.
4.19.2 - CL 4033788
4.20.3 - CL 4369336
Result: The dispatcher is no longer a dispatcher, and anything previously referencing the dispatcher will fail to compile.
Expected result: Cannot change the dispatcher type, leaving it as a Multicast Delegate.
There's no existing public thread on this issue, so head over toAnswerHub just mention UE-64694 in the post.