Many thanks for the reply. Absolutely got me pointed in the right direction.
I’m using XAMPP for my local staging environment and had to add the following to http.conf (in case anyone else runs into this), then reboot. Restarting apache didn’t trigger it for some reason.
<IfModule mime_module>
#
# TypesConfig points to the file containing the list of mappings from
# filename extension to MIME-type.
#
TypesConfig etc/mime.types
AddType image/svg+xml svg svgz
AddEncoding gzip svgz