There's some CSS that appears to be trying to make the modal close button smaller on mobile devices.

Most browsers seem to just ignore this, but Safari tries to follow it, and as a result cuts off the X used to close the modal since it extends beyond the size set. I'm not sure making this button smaller is a good idea, since people might be trying to hit it with their finger on mobile devices.

Possibly just deleting this line is the right fix?

.moodle-dialogue-base .moodle-dialogue-fullscreen .closebutton