I just tried it in both views, because I normally use the one with a seperate Tab Bar. There is no problem pulling the tab back into the window. I did exactly what's in the video, I think...
And this only happens when you drag it to the left side and this tlle frame appears before?
Maybe it is fixed it Safari 18.3 or it's another setting producing it.
You could also try if it is happening too with Safari Technology Preview, it's the same 18.2 version but a newer build.
The stand-alone betas for Safari 18.3 are only available for Ventura and Sonoma.