Yeah, that's not a regurge. A regurge woukd be partially digested and even with hay fever you'd smell it.
Based on the info at hand. I would say the feeders are bad. And honestly, I'd be reluctant to buy from that source again. Unless of course they were stored improperly once you got them?
Do you have another prey source??
I'd also wait 2 weeks to offer again. Your snake is at a weight that can easily handle a few weeks without...
BUT...you mentioned the snake is already losing weight??? Why is that???