Haraella retrocalla with yellowing midstem leaves
 

So the oldest and the newest leaves look hale and healthy, but it's those middle-aged ones that are in crisis!

I noticed one of them looking a little chlorotic a few weeks after its first bloom for me, so I cut back on light and paid more attention to watering. I was rewarded with lots of new root growth and 3 new spikes for a total of 4, but the yellowing continues and has progressed to obvious withering. It's pretty evenly distributed across the affected leaves, which is to say there aren't any spots or patches. All midstem, all right around the new root and spike growth.

The plant is housed in an open terrarium with natural west-facing window light in summer, and a supplemental light over winter. Fertilized every 2-4 weeks.

As an aside, I feel like 4 spikes at once is overachieving and I'll be happy to nip some off if that's advised.

Don't snip the spikes. They bloom repeatedly, sequentially. I'd just let the yellowing leaves fall, and observe the plant. There may be been something that it didn't like when those leaves were developing, and so it decided to shed them. But if the new growth looks good, it's happy now, keep on doing what you're doing and let it bloom. Enjoy, and note that it is delightfully fragrant. The plant will "pace" its blooming (probably won't do all a once) so it won't give itself any more stress than it wants.

Thanks Roberta. I'll stay the course.