Cusom visual z-index does not render correctly on IE/Edge

I have discovered an issue with custom visuals within Power BI Reports when viewed in Microsoft's IE/Edge browsers.
This affects all custom visuals (both from the store or built by us) so is not something that we have introduced via our custom visual code.

Reproduction Steps:
1) Add a custom drop-down filter or any other visual which expands/collapses.
2) Give the visual enough space (Style Width & Height) in which to expand.
3) Add another visual (any will do) below the custom visual so that its containing box overlaps the one above.
4) Set the z-index/layer which holds the custom visual to the back of the report so that hover/tool-tips/interactions with the other visual is not impaired.

The Issue:
In Chrome/Firefox/Safari - focusing on the custom visual will bring the visual to the top layer, allowing the rendering of any expanding content to show.
In IE/Edge - focusing on the custom visual fails to bring the visual to the front of the report. Any expanding content is hidden behind other visuals.

Notes:
Visuals obtain CSS rules for forcing a z-index.

.explorationContainer.reading .exploreCanvas .visualContainer.selected:hover:not(.popOut) {
z-index: 100001!important;
}

Custom visuals have their parent 'VisualContainer' div element modified correctly, but the rule is not applied to them, resulting in the 'z-index: 100001!important;' having no effect on the visual.

This rule works and is applied correctly in all other browsers tested by us so far. (Chrome/Firefox on Windows + Chrome/Safari on Mac + Safari on iOS)

Please can this be investigated as it is causing us major issues with clients who use IE & Edge.

Thanks,
Matt