strange!, why pacman -Syu still reporting this?

:: installing qt5-base (5.15.2+kde+r172-1) breaks dependency 'qt5-base=5.15.2' required by qt5-styleplugins

That's because qt5-base was updated again on Arch, from 5.15.2+kde+r171-2 to 5.15.2+kde+r172-1
As such, qt5-styleplugins needs to be rebuilt again.

installing qt5-base (5.15.2+kde+r172-1) breaks dependency 'qt5-base=5.15.2+kde+r171-2' required by qt5-styleplugins

Yet another rebuild is needed. qt5-base has been updated again on Arch:
5.15.2+kde+r172-1 => 5.15.2+kde+r172-2

rebuilt

And another. qt5-base has gone from 5.15.2+kde+r172-2 to 5.15.2+kde+r175-1 today.

Yet again:
5.15.2+kde+r177-1 => 5.15.2+kde+r180-1

qt5-base has been updated from 5.15.2 to 5.15.3
A new rebuild of qt5-styleplugins is needed:

installing qt5-base (5.15.3+kde+r132-1) breaks dependency 'qt5-base<5.15.3' required by qt5-styleplugins

The time has come again: qt5-base has been updated from 5.15.3 to 5.15.4
A new rebuild of qt5-styleplugins is needed.

installing qt5-base (5.15.4+kde+r135-1) breaks dependency 'qt5-base<5.15.4' required by qt5-styleplugins

A new rebuild is needed:

installing qt5-base (5.15.5+kde+r163-1) breaks dependency 'qt5-base<5.15.5' required by qt5-styleplugins

Yet another rebuild needed:

installing qt5-base (5.15.6+kde+r165-1) breaks dependency 'qt5-base<5.15.6' required by qt5-styleplugins

(Thanks for keeping it bill-auger!)

all set

Another rebuild is needed:

installing qt5-base (5.15.8+kde+r157-1) breaks dependency 'qt5-base<5.15.8' required by qt5-styleplugins

Thanks!

Yet another rebuild please:

installing qt5-base (5.15.9+kde+r151-1) breaks dependency 'qt5-base<5.15.9' required by qt5-styleplugins