Is your iPhone later than an iPhone 12 mini? If so, please send it to me!
I’ve had iPhones for quite a few years and as best as I can remember, they have all worked the same way when it comes to expanding the font on a webpage plus what happens when you hold the phone in portrait versus landscape.
I’m sure it’s not a phone problem.
The sudden, tiny font size on FR has happened a few times in the past but was later fixed.
Unfortunately, it seems, this time nobody at the FR end has looked into it.
It can make it hard and it takes a lot of zooming in & zooming out to be able to read threads if you’re on your phone.