On my Moto X4 running Android 9, there's enough room for 3 digits, so this appears to be browser or device specific.

Using a longer example (https://en.wikipedia.org/w/index.php?title=User:RoySmith/sandbox&oldid=1138014494#Heading), on the X4, it truncates at 3 digits.

Both of my examples are using Chrome.

With the explicit useskin=vector URL, the high-order digits overflow onto the grey left-side navbar, but they're visible on both the 6a and the X4.

@RoySmith I also cannot replicate. Which gadgets do you have enabled? Does the problem show up on https://en.wikipedia.org/w/index.php?title=User:RoySmith/sandbox&oldid=11380144947&safemode=1#Heading ?

On the phone, it's my mobile account, which is totally generic. I see the same issue logged out in an incognito window.

"The revision #11380144947 of the page named "User:RoySmith/sandbox" does not exist."

I can even replicate it on my desktop, using Chrome's mobile device emulation. For example, looking at https://en.wikipedia.org/w/index.php?title=User:RoySmith/sandbox&oldid=1138008943&useskin=vector2022, emulating a Galaxy Fold at 280 x 653 pixels. This one is using my normal (User:RoySmith) account, MacOS 12.6.1 Chrome Version 109.0.5414.119 (Official Build) (x86_64)

It comes and goes as I try emulating different device types. As far as I know, the Chrome mobile emulation just sets the appropriate screen size for the device, nothing else. So I guess this is really just a function of screen width.