The message is correct: the original package XX_PO_RCV_TXN_LABEL has indeed been invalidated when you have recompiled it. The business event turns to an old version pf the package, ... but it has been flagged as invalid in the share pool. Even if you recompile, or even drop the package, the workflow system will still turn to the old package version.
What to do: restarting the application services will definitely solve the issue (providing, of course, that the new package version is in valid state). For a production system this will cause downtime, so you could try to restart the Workflow Deferred Notification Agent Listener and perhaps the Workflow Notification Mailer.