I've seen this pattern in nearly every shop with qa/qe and engineers.  My advice:

- this is a practice that programmers actually do with each other.  Did you check this?  what about that?  What about the other?  It's not "questioning the person" though (although it certainly can feel like it), it's going through the assumptions and facts together and seeing what we have.  I have seen non-programmers not be able to handle this well as they don't expect it or understand it very well

- patience.  I see my role as QE (Quality Engineer) is to bring up _potential_ issues or things that _might_ be of concern.  Don't be alarmed if only 1/4 of them are real bugs or issues.  Don't think of it as 75% rejection.  Think of it as 25% success and if you didn't find that 25% the company would likely have lost customers / revenue, etc.  You may be surprised to find out how much the devs value to 25%.

- communication.  It is _essential_ to have good communication.  With managers and peers I recommend a 15-30 minute 1:1 every week.  For 'collaborators' such as the position you describe I recommend monthly 1:1's  During these meetings (may  not the first!) you can start to bring up the issues you mention.  However I recommend you do this **one issue at a time**.  Don't turn the whole session into a complaining one.  Try to bring up several positive points or other work details and then have 1 "why do we/you do this and how can we do it better".  
You should _also_ have regular communciation with _your_ boss and ask them to help.  At some point up the chain is a managerial position such as director or vp that overlooks both of you.  You may need to appeal to that person.