The reason why the traffic doesn't split 50/50 when using the Subgroup Round Robin Sticky test strategy is because your page has some components on the page with personalization rules.
This is how Subgroup Round Robin Sticky test strategy is described on the official Sitecore documentation:
The Subgroup Round Robin Sticky test strategy is similar to the ...
Turns out this feature is supported. In the Test environment where it wasn't working it was related to the Azure WAF blocking the content for some reason. Then it worked fine in UAT and Production (which was under a different WAF).