Issue on auto zoom while filming 0,5

Hello

I am having an issue why my iPhone 11pro while filming in 0.5.

i filmed a 22min film in 4K in 0.5 of me doing yoga. When I stopped the recording and checked the film, I noticed that my iPhone had changed the ratio progressively until it reached a final ratio of 0.7. I would like it to stay in 0.5 and not change at all.

I am filming myself so I cannot control what happens exactly when it happens.

Is there an option I should select not to have this filming ratio auto change while recording?

thanks

yann