I’m not sure why the link keeps changing to the subs link.

I submitted and verified that the link was: https://ig.ft.com/us-iphone/

But now it has changed?

It's because that link redirects to the subs link (presumably unless you have a subscription).

I think you can override it by changing the link shortly after submitting? But of course everyone who isn't a FT subscriber will get redirected anyway.