It looks that this Feature Flag - Enable experimental network loader is enabled by default, even when it is disabled. When you enable and disable it, then it is turned off. With this feature enabled, uploads from Mobile Network doesn't work, so It is a bug with this Feature Flag.

I found how to get it working using this WA, but it is not fixed in current IOS 18 Beta 7. To get it working, I went to Safari Feature Flags, and enabled and disabled flag - Enable experimental network loader. After that, uploads started to work. From this I conclude, that it is some issue with Safari network loader. After resetting to default settings, It again didn't work, so this is workaround.

Forgot to mention that on other browsers it works well both on WiFI and mobile network. My feedback case number is FB15233370.

Hey all, I have also provided feedback in IOS. There is such app in beta IOS versions. With Build 5 it is still not working, and my feedback case still is open. @BartFranse @PolU3417 guys If possible submit also feedback in IOS so they pick this ugly issue as soon as possible.