Since my last post, the new Android developer for NewsBlur has discovered the problem and identified a possible solution:
I tried it, and it and it seems to work. Unfortunately, there are other issues it doesn't fix:
Oh, well. At least the combination of Feedly and the FeedMe app is handling my RSS needs admirably for the time being.
Edit, one week later (2019-05-15): Still no resolution on this issue. I have nine days left to decide, but I think I'm going to stick with Feedly and bid adieu to NewsBlur due to their completely inadequate, hapless, and possibly incompetent response to this issue. Time to move on.