Old PLT file?

Hi everyone,

We are doing our CI on github. We cache the PLT so that we don't have to 
rebuild it every time, and this mostly works fine, but occasionally 
dialyzer (or, rather, "mix dialyzer", as this is an Elixir/Phoenix 
application) determines that the PLT is old and the workflow is aborted.

My question is, what makes a PLT file "old"? We very seldom change 
versions of OTP or Elixir, nor do our dependency versions change very 
often, yet we get into this situation a lot more often than I believe we 
should. If I could understand a little better about what makes a PLT 
old, perhaps I could solve this issue.

Thanks,

Michael Martin