Hi. Sorry for the late reply. I tried replicating the issue on my end, but doing the same setup (or at least what I can suppose from your comment) yielded results that looked correct to me. My best guess for the issue is that something about how you defined the sprites is causing renpy to reset the anim_time value, which would cause the sprite_highlight function to snap to the highlighted value. Which maybe you're confusing for the .2 seconds given .2 is pretty fast. Though I could be wrong. This is just my best guess given the information.
I tested with NVL and that didn't seem to make much of a difference. If you'd want to show me your code so I could test it and see what the issue might be, I'd be happy to take a look when I have time. (My username on discord is wattson if you need to reach out). But yeah I'm not entirely sure what the problem might be beyond my guess. Hope this was helpful and a solution to your issue can be found!