Many thanks for this thread. It was very helpful and there seems to be very little about this on the web.
I've been meaning to work out why this broke for a while and today was the day!
I hope you don't mind me adding to it so long after the last post but my Qnap is on version 18.104.22.1681 and the feature still is absent so here is a workaround for the interim.
From the diagnosis left by @ramblingpolak , I have created a script for Tampermonkey which is a browser extension that edits html on the fly.
Tampermonkey should be available on your browser extensions page.
Add this script, adding your nas details. I use a custom port but I think the default is 443 for HTTPS and 8080 for HTTP?
It just changes the tags to tags and given the right include instructions, will only run on the specific webpages.
I am afraid it is very crude but I am not a coder.
The tags are only used once on my qnap but I believe you can add things to the login screen which might use it and be effected.
I did try to get it to replace the whole line but I got mixed results.
Maybe someone who knows what they are doing could suggests improvements.
I just wanted to post this here for the benefit of those in the thread and those who search the thread who clearly find this rather annoying.
I am using Firefox and I haven't tested it with other browsers.
Also, I use a different password manager so I haven't tested it with 1password either.
It is such a simple fix that there is no reason to think it shouldn't work with any of the mainstream combinations.
Hopefully it a fix will come out soon.
I'm afraid I can't work out how to stop the forum page trying to format the code but it still comes out useable.
// @name Fix Qnap page
// @namespace http://nas-address
// @version 0.2
// @description User Upgrade
// @author Me
// @include http://nas-address:nas-html-port/*
// @include https://nas-address:nas-html-port/*
// @grant none
// Replace textarea with input in the HTML to be compatible with password managers
Thanks for the additional information @Dennis_J and for sharing your script. I believe I've fixed the formatting for you. For future reference there is a "code" option under the paragraph formatting selection: