That’s why this forum is great. It’s not about how smart you are when it comes to teaching the swing. Someone might see something that you don’t. Don’t worry there have only been a few who pushed my buttons in the past.You are (subtly) shifting your weight first to your front leg. You can see it when the front knee moves first. It is leading you to a front-leg block/push-back which causes the head popping up (vs. lowering as you'd see in a live, well-sequenced swing). I don't mean any of that as an insult rather a description. It is somewhat understandable as you are really concentrating on the top half, not the bottom. So you are getting more pushback than clear the hips action on the front-side. And when that happens, as in many low-effort swings, the launch is much more against and around the front-leg. You see it in soft-toss, tee swings, warmup circles swings, swing demos, etc. You know, the things some build their swing models around.
Try playing around with this: move closer to the wall, putting the bag deeper in your stance. Loosen your grip to allow a more handle-torquey rearward TTB feel in your hands. IMO you are death-gripping a bit. Let the front arm ride more up than around. On the bottom half, play around with an open or three quarters stretch and fire set-up position with a strong rear-leg-weighted/rear hip pivot point feel to the launch. See what it does your your tilt, your posture and head rise, bat path and contact point, which should now be deeper and more next or behind you than out in front. Different feels all around.