It seems that the rendering of an incomplete .exr file display tiles with noise instead of background color (or black pixel with null alpha).
I tried with multiple rv versions and I saw that the problem appears between rv 7.3.0 and rv 7.3.1.
I was able to reproduce the issue which was introduced in RV 7.3.1 as you correctly determined. I have a fix for it which I have to send through the review and QA process. Once the testing is done, I could make a Pre-Release build available to you sometime next week if you wish.