SBT (due to using Coursier since 1.3.x) will by default hold on to the old cached snapshot for 24 hours. You will need to set COURSIER_TTL=0s (as per SBT docs: https://www.scala-sbt.org/release/docs/Dependency-Management-Flow.html#Notes+on+SNAPSHOTs):

export COURSIER_TTL=0s
sbt update # <- this will now always lookup for changes