Remove Immutable Tracks import option breaks some character animations

[Illauriel]

Tested versions

Reproduces in: v4.3.dev.custom [7d151c8]
Doesn't reproduce in: v4.2.1.stable

System information

Win 11, but it's not an OS issue

Issue description

When importing animation files with single animation, where armature's initial pose is the first frame of the animation, Remove Immutable Tracks option removes one-key tracks that set this pose up, causing bones affected by these tracks to show rest or previous pose instead of the correct pose. This is especially noticeable on character hands holding weapons and staying in one pose throughout the animation clip.

Screenshots: Left - Remove Immutable Tracks enabled, Right - disabled

I've mostly encountered this on skinless one-clip animations FBX files exported from 3dMax, common in animation packs from asset stores. I tried to reproduce this in 4.2 and didn't manage to find a file that would import but have this issue. Probably means it's not a regression. It wasn't such a problem in previous versions, because previously such files would just be impossible to import :)

Steps to reproduce

• Add animation file to project
• Tick Import as Skeleton Bones if necessary (see Add new scene import option to import as Skeleton #88819)
• Set Rest Pose to an external T-pose animation fit for this rig (see Retargeting option to use a template for silhouette. #88824)
• Set BoneMap
• Reimport

Remove Immutable Tracks is on by default, so this issue pops up during normal import process. You don't need to go out of your way to encounter it. Note that it happens on specific files that would not import correctly without the new process.

Minimal reproduction project (MRP)

I encountered this issue with commercially available asset packs, so I can't really include them in an MRP and I was unable to reproduce them with mixamo animations because they have a proper initial pose baked into them, even if you download the versions without the mesh.

EDIT(lyuma): I made a blend file to reproduce the issue (rename the file extensions after downloading):
simple_vrm_rig_anim4.blend
simple_vrm_rig_nlaonly2.fbx