Why not enable Fixed-Foveated Rendering for intermediate render texture?

I found Fixed-Foveated Rendering works well when I don't use any render feature or post process.

But if I add any pass requires rendering into a intermediate render texture, then FFR is disabled for those render passes and only enable in FinalBlit pass.

I have checked the document and it also said not recommend to use FFR in this situation:https://docs.unity3d.com/Packages/com.unity.xr.oculus@4.1/manual/index.html

I also checked the Vulkan document and found nothing force me not to use FFR in render pass which uses intermediate render texture. Is there any options could force enable FFR or am I missing something which not allow FFR being used in this case?