Per the comment by @markalex, the issue here was that my buckets for the histogram topped out at 10000, so when the value was above that there was no way for the quantile to show this. I've adjusted the buckets to more accurately cover the expected ranges for the value, and now everything looks better.

Some good resources (also provided by @markalex) to see how the histogram_quantile function operates are:

Prometheus documentation on errors in quantile estimation (I was seeing an extreme case of this)

This answer by @Ace with good detail on how exactly the histogram_quantile function operates.