It may be a little late, seeing that it was answered almost a year ago ... Although I tried your meta tags on my HTML page of my iOS application. The tags really did not allow me to zoom out, and I could hardly zoom in, and when I zoom in, it will go back to its original size.
Now I’ve managed to completely zoom out and zoom in a lot, and it will keep scaling using these meta tags:
<meta name="viewport" content="width=device-width, initial-scale=.5, maximum-scale=12.0, minimum-scale=.25, user-scalable=yes"/>
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