I did not use a snapshot, but like @anon2180415 I used the official ImageBuilder for 21.02.0-rc2 (and I waited until the release showed up on the front page). And this seems to be the culprit. Because my image has the package version git-21.147.37148-a86e770 of luci-mod-network which does not include your fix. There is however a newer version of that package available via opkg. So, I will try that again later. Looking at the openwrt-21.02.0-rc2-ath79-generic.manifest file, I can also see that the official image comes with a newer version of luci-mod-network, namely git-21.149.58794-09329fe which includes your fix.
In any case, I'm a bit puzzled now because I always assumed the image builder would generate images with the same packages as the official images (or at least not older than that). But that doesn't seem to be true, apparently. 