Found the answer. It is a bug in WordPress v5.2.3 affecting WordPress in Windows environments. Caused by backslashes in Windows paths that aren't stripped correctly
Details here: https://core.trac.wordpress.org/ticket/47980
I have tried the patch suggested and can confirm it solves the problem.
$location = '/' . ltrim( $path . '/', '/' ) . $...
Seems like a good list you have going... Just a few that come to mind:
SCRIPT_DEBUG to false on live and true on staging.
WP_CACHE to true on live and false on staging.
ping_sites to empty on staging to disable ping services.
default_pingback_flag to 0 on staging to not send outgoing pingbacks.
Maybe install a Password Protect plugin on staging (in mu-...