Thanks Steven for your swift reply !
All software is update.
Indeed it seems to be the "internal browser of messenger" the cause.
Using the link via Mobile Safari works just fine.
I have reproduced the issue with your gallery https://www.juicebox.net/demos/pro/default/
Sent link via messenger
Clic link in messenger.
(phone is held stand : vertical)
Thumbnails are displayed fine
Clic first pic
Tilt screen horizontal to display the picture larger (as its a horizontal one)
The picture and screen are as expected
Swipe to display next photo (still holding phone horizontal)
The picture and screen are as expected
Tilt screen vertical to display the picture larger (as its a vertical one)
-->> only a half screen is displayed.
Tilt back to get out of this "half screen".
So the issue is vertical photos can not be displayed in "large"
Screen shot attached