Minimal example:

case class A(a: Int = 1)

import upickle.default._
implicit val rw: ReadWriter[A] = macroRW[A]

println(read[A]("{\"a\":2}"))
println(read[A]("{}"))

Expected (upickle 1.4.0, Scala 2.13.6):

A(2)
A(1)

Got (upickle 1.4.0, Scala 3.0.1):

A(2)
A(2)

God knows how many projects are silently corrupting their databases. Or leaking data from another user in their API calls. Since the parser is keeping and leaking state, everything is possible.