When user role is updated, last_update user meta should be updated as well.

[poglaa]

When a user is saved and roles are changed, I believe last_update user meta should be updated as well.

[bradparbs]

I may be mistaken, but as far as I can tell, Core doesn't use the last_update user meta key at all. I see that WooCommerce does set it, so that might be where you're seeing it. Unlike the other WC meta fields, it doesn't have a prefix.