Not an expert, but my first reaction of photo (before reading text) was "That bike looks small for him".

But, if the fit is same as the other bike that is kinda weird.

Are tires too highly inflated?

I don't consider myself an expert, but... It looks kind of like you're elbows are a bit rigid, but not quite locked out. I'm wondering if you're pushing yourself backwards with rigid elbows and thus arching your lower back. Try relaxing the elbows.

But if the last bike felt great, but this one causes problems, then it almost has to be that something was lost in the transition from one to the other. The position must not have been duplicated exactly.

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