That's where my money's at. Usually if you have a rendering error in an otherwise competent product, it's due to it being programmed for a different domain. Here, the CPU and GPU have such different types of work units that structuring for GPU efficiency often means making poor structures for CPU efficiency. Said another way: writing it to be performant on the GPU usually means the data would be hard to work on quickly with a CPU.

Add in to the mix that the rendering is not (usually) being managed explicitly by the implementer and is, instead, being left to browser implementations, along with the issue only being a problem for people that don't have discrete GPUs, and you've got a recipe for real thorn in your side, as far as bugs go.

Rewrite the whole render stack due to a performance issues for a subset of users?

Bifurcate the rendering based on integrated vs discrete GPUs?

Bug the browsers that seem to be mishandling CSS animations on integrated GPUs, and then hope they care?

Not a lot of great options there. But here's to hoping they figure something out! Solving a hard problem feels pretty good, even if it's not appreciated by your full demographic.