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I have finally discovered the source of this issue and how to fix.
Some of the confusion was caused by the fact that, since my app is still in early development it was actually loading Vuefire from CDN rather than from a local dependency.
Recently Vuefire devs have changed from 1.x to 2.x as their official stable release. That said, it would seem that in 2.x (now the stable channel) and 3.x (next pre-release candidate) releases, with the main focus shifting to Cloudstore, there are now major problems with Realtime Database implementations.
After downgrading Vuefire to version 1.4.5 (latest 1.x release) all of my original source code is once again working as expected.
Source:stackexchange.com