Videos don’t run on iPhone

lately all the videos I export don't run on iPhone XS. My videos are always the same codec. I export mp4, or QuickTime, with the "YouTube tab" or with the "custom settings" tab (h264, quality restrict 3500). On I phone works only the audio, the video is not working or is very lagging. I also noticed that these videos (that I upload on my socials using the browser, due to the fact that I cannot watch it on iPhone because as I said they don't work on mobile) randomly don't run properly in the social media. They are like damaged: the audio of those videos in the social media stop working for couple of seconds. Apple support says is Davinci problem.
I tried it with davinci 16 and 17 beta.

Ps
I don’t want to export with h265 because social media don’t recommend it and I don’t want to use any external app like handbrake because it makes my workflow slower and I saw a small decrease of resolution.

I hope someone can help me.
UPDATE with handbrake I see a huge decrease of resolution especially for videos with solid colours and small parts. So, I definitely don't want to use handbrake.
I want that davinci is reliable as it was months ago. I want to be able to play my exported videos also on iPhone