Imposter sprites don't rotate if too many in an instance?

Background: I’ve created a grassy meadow in my game, full of flowers. The flowers are an imposter sprite, slapped onto a simple static mesh plane. I’ve used the Foliage Paint tool to paint this mesh onto the terrain in the level. The result is an InstancedFoliageActor with hundreds of flowers in it.

The problem: Unfortunately, the flowers do not rotate around the Z-axis like Imposter sprites are supposed to - they just stay in the same rotation frame forever. I’ve noticed that this happens only if there are many of them present (there are possibly over a hundred on this meadow). If I only add a few (like a dozen or less) with the Foliage Paint tool, they DO rotate as Imposter sprites should. I’ve replicated this in a completely different map and on a different model than the meadow. The problem also happens in the UE4 editor viewport, when the game is not running.

The sprite does work as it should when I am looking at its Imposter material in the UE4 material editor. It also works fine if it’s just a plain, single static mesh, not instanced.

Is this an intentional limitation in the engine? Does the engine simply not support more than a dozen or so instances of an Imposter sprite? Or is this a bug? Maybe I’ve misunderstood how instances are supposed to work.

I am using the latest UE4 version available through the Epic Launcher. 4.11.1

Still happens in 4.18. Epic?