Webm with alpha

I have installed the webm plugin from fnord and it appears in the media encoder, however the option to include the alpha channel doesn't seem to work. I have no idea why.

I use OSx 10.14.6 Mojave 

Premiere Pro v14.3.2

Media Encoder v14.3.1

I've spent 2 days trying to find a solution!!!

Can anybody help please?

Why is webm not an option included with Adobe Media Encoder in the first place?