iPhone 15 Pro Display Brightness limitation - often not increasing brightness above 80%, even when phone is not warm

My iPhone 15 Pro appears to have a Display Brightness limitation issue that’s incredibly frustrating.

The brightness does progressively increase when sliding the display brightness ‘slider’ up to around 80%, but beyond this there is no increase in brightness.

This problem exists even when the phone is ‘not’ warm (the phone can be quite cool).

Most importantly, is that powering ‘off’ the phone and then powering it back ‘on’ instantly resolves the issue and brightens progressively increases all the way when sliding the brightness slider up to 100% (max) and the display is notably brighter than before. However this is only a brief fix, as the problem returns.

Note:-

I permanently have Auto Brightness set to ‘off’. I don’t like it and never have, as the brightness in low light environments is automatically set too low for my preference. Auto brightness on many devices appear to aggressive in low light conditions - if the user could fully customise the Auto Brightness behaviour, then this would be a first.

Night Shift is always off. I personally don’t require it and have no need for it… though having the option there is great and is handy should I ever need it.

True Tone setting makes no difference to the issue relating to this post. I have used the phone for extensive periods with this feature both enabled and disabled, but the problem of the display brightness being capped still exists. My preference on a side note, is usually for True Tone to be set to ‘off’, because when enabled it can at times produce a very subtle yellow’ish tinge being almost a urine-colour tint in appearance. With True Tone turned off, I find the white colours to be more of a natural white.

I have played with “Colour Filters”, but again this has no effect on the display brightness cap experienced (at around 80% brightness).

Just to re-iterate: rebooting appears to temporarily resolve the issue - but only temporarily.