'Page URL not defined' for lots of iOS users

We’re getting ‘Page URL not defined’ recorded for a lot of users on the site (it’s been increasing recently). These users are exclusively using iOS devices.

There seems to be no trend with OS version or device type. The browser is recorded as unknown. Some iOS devices are being tracked fine, so this doesn’t affect all iOS devices. But it’s affecting lots (perhaps most) of them.

Can anyone offer some pointers on what might be going wrong, and what we might be able to do to fix this issue?