Other than the spaces in the URL - which I assume are because of auto-formatting in the post - that looks correct. There are a few places in the API where I’ve found things just don’t work as expected. Report it up as a bug.
Can you use /PODetails({Company},{PONUM},{POLine}) instead?
This is an old post but, in case anybody like me came to the forums after banging their head against the wall, I thought I would at least note what was required to resolve this in my case. In my case I was trying to do something like this (URL encoding omitted here for readability):
/Erp.BO.ResourceSvc/Resources?$filter=ResourceGrpID ne '9999' and ResourceGrpID ne '9998'
I could not see what could have possibly been wrong with that. Then it hit me that I had a BPM in place that looked like this:
In my case here, Epicor was doing exactly what I was telling it to do - I was overriding the $filter (GetRows whereClauseResource) with what was in my BPM. Doh!