Update: I discovered by testing other classes that the subtree_array and the filter path:node_id both return the root node given in the results. So the problem seems to be that my project class objects aren't being properly indexed for some reason.
I discovered that the problem related to the presence of an object relations attribute in the project class. For some reason when objects had an item selected in the object relations attribute, they weren't indexed properly. I'm using http://projects.ez.no/ezfsolrdocumentfieldobjectrelation/ so perhaps that has something to do with it. I solved the problem by editing all the project objects to de-select the object relation, submit, re-edit, re-select and re-submit. After this the objects were all found and returned by the ezfind search.
Thanks for keeping up posted along the way, and good to hear you found a solution. I was to recommend to surface the issues you encountered directly on the project's page, but could notice you already did so on other subjects.
How about requesting membership to the project and bringing updates, based on your field experience ?
Cheers !
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Nicolas Pastorino
Director Community - eZ
Member of the Community Project Board