The legacy router relies on the master-service route to determine its pathing.

The PE route in /etc/puppetlabs/puppetserver/conf.d/web-routes.conf web-router-service section is:

"puppetlabs.enterprise.services.master.master-service/pe-master-service": "/puppet"

But the legacy router is searching this config for:

"puppetlabs.services.master.master-service/master-service": "/puppet"

Because this route is not present in PE, re-routing of legacy urls is failing like so:

curl --cacert /etc/puppetlabs/puppet/ssl/ca/ca_crt.pem --cert /etc/puppetlabs/puppet/ssl/certs/pe-399nightly-master.puppetdebug.vlan.pem --key /etc/puppetlabs/pupp

et/ssl/private_keys/pe-399nightly-master.puppetdebug.vlan.pem https://pe-399nightly-master.puppetdebug.vlan:8140/production/resource_type/bar

{"message":"Not Found: No route for GET /v3/resource_type/bar","issue_kind":"HANDLER_NOT_FOUND"}

The work around jeremy.barlow suggested is to add the master-service route for present so that the legacy router has something to reference.