I have been using Osmo+.
The focus fits well on the wide angle.
But when I entered the zoom point, I noticed that there was a mismatch in focus.
Looks like a problem with backfocus.
How can I solve this problem?
How to adjust back focus?

I always have to refocus after zooming. I've always considered the zoom to be for re-framing a shot. I never use it live. It's just not responsive enough.

There is no back focus because there is no focal flange. Backfocus only applies to removable lenses where you change the distance from the rear element of the lens to to focal plane. ie sensor.

The Osmo+ lens is not parfocal which is what you seem to be wanting. One that you zoom in and focus and then stays in focus through the entire range as you zoom out.

A lens like that on the Osmo  would be awesome but is generally only found on much higher end broadcast and cine equipment. As the other poster said....the zoom is only really for framing.