Actually, this is normal; you should realise that while the UXP platform was forked from a Firefox ESR 52.6.0 platform snapshot, the application Pale Moon 28 itself (built on the UXP platform) does not implement the Australis GUI, much like its previous major version 27.
The feature you describe was added on top of the Australis interface (Fx >=29.0) by Mozilla developers and should be there in both Firefox ESR 52.9.0 / Basilisk 52 (Serpent 52.9.0), as long as you do not move the "Zoom Controls" button (- +) to a browser toolbar; this native browser feature was not present in the pre-Australis (Fx <=28.0) Firefox GUI that PM uses...
There's nothing to fix here, as explained above; BTW, you should not expect @roytam1 to implement major new features in New Moon 28 (his fork) that are not yet present upstream, i.e. in original Pale Moon 28 source (master branch); the way I see it (and please someone correct me if I am wrong), Roy's main focus is/should be creating and testing patches to Pale Moon that will enable it to run on Windows XP + Vista, both OSs that the official app has dropped support for; main browser code is still the task for Moonchild Productions; if you do find a bug inside Roy's patches (that enable XP+Vista support), by all means report it here, but GUI improvements + implementation of new features should not, IMHO, be a thing for Roy...
In any case, if you want the requested functionality in New Moon 28 (currently at version 28.2.0a1), you can install an extension which offers pretty much the same thing, Zoom Page v15.7 (it creates a button that can be placed inside any toolbar, but, sadly, not inside the URLbar):
I think it's universally accepted there's no software that is bug-free; however, your bug reporting skills are very flimsy in this case; please read Information to include when asking for support ; providing just a screengrab without further info (e.g. the URL details of the page where this occurs) so other people here can test and try to reproduce results in pure sensationalism, with slim chances of a resolution of your issue...
Please understand I have nothing personal against you , my advice above was simply offered so us members here can help each other in a more productive manner...
Regards