When applying PropertyFilter to an Entity, properties is exposed as a simple string, whereas it should be a string collection. As of now implementation is broken, ie, you can't ask for more than one property if one were to adhere to the documentation.

It's now detailed in the documentation as:
{"name":"properties[]","in":"query","required":false,"type":"string"}

Should be:
{"name":"properties[]","in":"query","required":false,"type":"array","items":{"type":"string"},"collectionFormat":"multi"}