'Why some http requests use http/2.0 and others use http3?

In my nginx server's access log, I saw that some requests were using http/2.0 or http/3 even from the same device.

Requests are made with an iPhone and iOS 15.3, but weirdly (for me) the http version changes at 30 second intervals. access log

I need to figure out why, because POST, PUT or DELETE calls fail with HTTP/3.

Any idea ?

ios http nginx

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