Thanks for your kind welcome!
Ah yes, I should probably have added that this has been happening for many years (probably as long as rocker gestures have been available) and across several OS/DE and hardware combinations, so I just assumed it would be happening to anyone.
At the moment I'm using Manjaro (6.8.5) with KDE Plasma 5.27.11 (X11), Vivaldi 6.7.3329.17 stable, and a Logitech MX Master 2S (no other problems with it).
To make absolutely sure that I hadn't botched anything on my installation, I just installed Vivaldi (from the official site, no extensions added or anything) on a Windows 10 laptop, which has never had Vivaldi installed before (and using a different mouse), and the same thing happens there.