Comment # 4 on bug 1208681 from Michael Hamilton

There is a background KDE service called GNOME/GTK Settings Synchronisations
Service, I suspect it is overwriting .config/xsettingsd/xsettingsd.conf.  I
tried manually setting the DPI in xsettingsd.conf.  When I login the config
file gets changed back to 98304, which means 96 dpi (98304/1024=96). 

I went to KDE settings->Workspace->Startup and Shutdown->Background Services
and disabled the KDE service, and now everything is back to normal.

I think KDE 5.27 might have been updated to make clever decisions about DPI,
and might be picking the lowest (or it's just getting it wrong) - I have two
monitors, one is 161 dpi and the other is 96.

I'm not actually sure what "correct" is, but I'm back to where I was.  

Even with the KDE service running, xdpyinfo is returning a resolution of
161x161 dots per inch, but perhaps the KDE service is looking at both my
monitors, one is 161 DPI and the other 96 DPI.

My login config files date back to 1992.  In case my user login and xrandr
settings were contributing to the problem, I did my debugging under a new
login.  That login  sees the same issue, so I think this problem might be
general to folk with monitors that have two different DPI's.