protection against illegal methods in legacy functions hides syntax error

Puppet Version: 6.4.0
Puppet Server Version: 6.3.0
OS Name/Version: CentOS 7.6.1810

After updating our Puppet Server to latest release we received this issue on agents:

root@xyz1000 # puppet agent --test --environment=production
Info: Using configured environment 'production'
Info: Retrieving pluginfacts
Info: Retrieving plugin
Info: Retrieving locales
Info: Loading facts
Error: Could not retrieve catalog from remote server: Error 500 on SERVER: Server Error: Evaluation Error: Error while evaluating a Function Call, The code loaded from /etc/puppetlabs/code/environments/production/modules/etchosts/lib/puppet/parser/functions/get_host_hash.rb does not seem to be a Puppet 3x API function - no 'newfunction' call. (file: /etc/puppetlabs/code/environments/production/modules/etchosts/manifests/base/solaris.pp, line: 94, column: 15) on node xyz1000.xyz.de
Warning: Not using cache on failed catalog
Error: Could not retrieve catalog; skipping run

The affected function is valid and was accepted valid before (I only replaced "values" by an empty array to reduce complexity:

module Puppet::Parser::Functions

  newfunction(:get_host_hash, :type => :rvalue, :doc => "") do |args|

    # start with an empty hash

    temp = Hash.new

    # empty array in this example

    values = Array.new

    # get the first argument number

    firstargnr = args[1].to_i

    # get the first argument number

    secondargnr = args[2].to_i

    # analyze each entry in the values

    values.each do |entry|

      # split the entry array to sub arrays

      subentry = entry.split

      # if first requested element starts with # sign or second requested argument starts with + sign skip this entry

      if not subentry[firstargnr].empty? and not subentry[firstargnr].start_with?("#") and and not subentry[secondargnr].empty? and not subentry[secondargnr].start_with?("+")

        # add to the hash

        temp.merge!({"#{subentry[firstargnr]}" => "#{subentry[secondargnr]}"})

      end

    end

    # return the hash

    temp

  end

end

Desired Behavior:

Works fine with 6.0.x and 5.5.x.

Actual Behavior:

Use of "func3x_check = false" in puppet.conf to get te catalog creation running again.

root@xyz1000 # puppet agent --test --environment=production
Info: Using configured environment 'production'
Info: Retrieving pluginfacts
Info: Retrieving plugin
Info: Retrieving locales
Info: Loading facts
Info: Caching catalog for xyz1000.xyz.de