I’m not 100% sure but I have a couple of ideas that might explain what you are noticing.
You mention that you have “climbed a mountain” prior to seeing this. I assume then that you have been going for several hours. It’s quite likely then that you have accumulated significant fatigue in your “uphill” muscles. So much so that when you start back up hill after a descent those main uphill propulsive muscle groups are prefatigued and so respond poorly to the demand os going up at a normal PE.
The second thing, which is also likely is that going downhill uses your leg muscles in a very different way than when they were carrying you up. Gravity is doing most the work on the way down and your muscles primarily resisting gravity using eccentric contractions. When you sudden switch to going up during along descent it is as if you are jumping off the couch with no warm up and trying to go up at the same pace as before but with no warm up.
Other than more aerobic base training I can not think of a way to improve this.