Interesting, the icon has a fill of #000000 (it is from modenuiicons). Dies the replacement happen on the client or the server side, because the base64 on the client already has the color of the first baked in for both. Also your demo does not seem to do anything.
The color change is done by the client taking advantage of the browser’s built-in svg/xml support. It’s a bug with icon packs. It looks like the color change updates the cached original so at the next change the #000000 is not there anymore (few builds ago wisej was always changing the monochrome color without checking for #000000 – that’s probably why it escaped). If you hit refresh after changing the color you’ll see that it’s changed: when using the icons in the icon pack it works only the first time, then the original is changed. You can also try with the theme icons (i.e. “icon-print”) in which case it always works. I’ll log and it should be fixed with the upcoming update, it’s an easy thing.
And the bug is present only for the ModernUI icons because instead of a global fill color they set the color for each path element, which is where wisej tries not to change to much and checks for #000000. If you try MaterialDesign or FontAwesome icons it works.