bikermondo This thread relates to a specific software issue that affects only the iPhone XR on the O2 network (and consequently giffgaff) where it periodically drops to 'No Signal' and needs to be powered off and on again to re-connect. It's been widely covered in the media. As discussed, Apple and O2 have acknowledged it, and Apple should be releasing a fix for this in the coming days. The iPhone 11 is not affected though. I am running an 11 Pro Max on giffgaff with no issues.
In the situation above, were both iPhones on the 4G network? Or is it possible that the iPhone SE was on the 3G network at the time of the image? It might be worth doing some side by side comparisons with wifi off so you can see the 3G/4G icon. I sometimes find that my phone will drop a very weak 4G signal in favour of a full strength 3G signal. If this is the case in the situation above, it would actually indicate that the 11 is holding onto a weak 4G signal for longer rather than dropping it in favour of 3G. This is just guessing though, without seeing the 3G/4G icon you can't tell.
The hardware designs are quite different between the SE and 11, so there could be some variation. I would not have expected so much though, and would have thought the design would have improved over the generations.
Also, as @nigelbrooks says, it may be worth contacting Apple if the variance persists.