Looking at the default rules for uMatrix:
https-strict: behind-the-scene false
matrix-off: about-scheme true
matrix-off: behind-the-scene true
matrix-off: chrome-extension-scheme true
matrix-off: chrome-scheme true
matrix-off: moz-extension-scheme true
matrix-off: opera-scheme true
matrix-off: wyciwyg-scheme true
referrer-spoof: behind-the-scene false

...and the default rules for uBlock:
about-scheme
chrome-extension-scheme
chrome-scheme
moz-extension-scheme
opera-scheme
vivaldi-scheme
wyciwyg-scheme

Shouldn't these be removed from uMatrix?:
https-strict: behind-the-scene false
matrix-off: behind-the-scene true
referrer-spoof: behind-the-scene false

...and this one added?:
matrix-off: vivaldi-scheme true