Hair distribution jumps about unwantedly

Hello!

I have a character’s head with hair growing from it. I use shape keys to deform the head, which i keyframe to change over time.

On most frames, the hair moves with the deforming mesh as you’d expect, but on 1 or 2 frames, the hairs jump, that is to say the layout from where each hair grows from changes. It is especially obvious if I am using clumping, because clumping makes a hair growth pattern more obvious.

Can anyone help with this please?

I managed to work around this, but not fix it. I had to make sure that the shape key changing the shape of the mesh never exceed 0.996 of influence, and that way the hair growth layout did not suddenly change. If it went to 0.997 to 1.000, it would change! Any ideas why?