reproduction steps

val f1: PartialFunction[Integer, Nothing] = { case _: Integer => throw new RuntimeException("f1 blew up") }
val f2: PartialFunction[Integer, Nothing] = { case _: Integer => throw new RuntimeException("f2 blew up") }
f1.andThen(f2).isDefinedAt(3)

Using Scala 2.11.12, the last line returns true

Using Scala 2.13.6, the last line throws the first exception (java.lang.RuntimeException: f1 blew up)

problem

The behavior of isDefinedAt across versions is inconsistent.
Given that with both Scala versions f1.isDefinedAt(3) and f2.isDefinedAt(3) return true, I think that returning true is the correct result.