In Animate 2020 when I try to make a classic tween it ends up looking like this.
This happens to everything. I do it the same way as in the past versions., yet I get this.
there's not enough information in your post to help you. eg, that looks exactly like you should expect if you're tweening the width or xscale.
so to start you should show
1. an example of 2 keyframes that you're tweening and
2. the resulting tween and
3. explain how it's different from what you expect.