The bug

It looks like illager and golem statistics are not upgraded correctly. They are upgraded to the entity id used in previous 1.13 snapshots, but not the new ones introduced in 1.13-pre5.

Invalid statistic in ###\stats\0fec4f7a-6a9b-4c43-ad1b-235e7fbc9822.json: Don't know what minecraft:evocation_illager is
Invalid statistic in ###\stats\0fec4f7a-6a9b-4c43-ad1b-235e7fbc9822.json: Don't know what minecraft:vindication_illager is

Note: For some reason both messages are logged twice.