AssertionError on Scala 2.13.10 project when not using any assert statement - scala

Question

I'm getting the following error code when running sbt clean assembly on a Scala 2.13.10 project using JVM 1.8 and IntelliJ IDEA 2022.3.1 (Community Edition). I think it's not caused by any assert included in my code.
java.lang.AssertionError: assertion failed:
List(method apply$mcI$sp, method apply$mcI$sp)
while compiling: D:\repo\git\external\project9\src\main\scala\es\package\graph\KNNGraphBuilder.scala
during phase: globalPhase=specialize, enteringPhase=explicitouter
library version: version 2.13.10
compiler version: version 2.13.10
reconstructed args:
last tree to typer: Ident(x1)
tree position: line 270 of D:\repo\git\external\project9\src\main\scala\es\package\graph\KNNGraphBuilder.scala
tree tpe: x1.type
symbol: case value x1
symbol definition: case val x1: (Int, es.package.graph.NeighborsForElement) (a TermSymbol)
symbol package: es.package.graph
symbol owners: value x1 -> method $anonfun$addElements -> method addElements -> class GroupedNeighborsForElement
call site: method neighborsWithComparisonCountOfGroup in class GroupedNeighborsForElementWithComparisonCount in package graph
== Source file context for tree position ==
267
268 def addElements(n:GroupedNeighborsForElement):Unit=
269 {
270 for ((k,v) <- n.neighbors)
271 addElementsOfGroup(k,v)
272 }
273
n.neighbors is returning Map[Int,NeighborsForElement] here. Other relevant code can be found below:
def addElementsOfGroup(groupId:Int, n:NeighborsForElement):Unit=
{
getOrCreateGroup(groupId).addElements(n)
}
private def getOrCreateGroup(groupId:Int):NeighborsForElement=
{
val g=neighbors.get(groupId)
if (g.isDefined) return g.get
val newGroup=new NeighborsForElement(numNeighbors)
neighbors(groupId)=newGroup
return newGroup
}
This is the build.sbt file:
name := "project9"
version := "0.1"
organization := "es.package"
scalaVersion := "2.13.10"
val sparkVersion = "3.3.1"
resolvers ++= Seq(
"apache-snapshots" at "https://repository.apache.org/snapshots/"
)
libraryDependencies ++= Seq(
"org.apache.spark" %% "spark-core" % sparkVersion,
"org.apache.spark" %% "spark-mllib" % sparkVersion,
"org.scalatest" %% "scalatest" % "3.2.0"
)
assemblyMergeStrategy in assembly := {
case PathList("META-INF", xs # _*) => MergeStrategy.discard
case x => MergeStrategy.first
}
Any ideas what's wrong with this code? Thanks