As described in CMS-16060.
Any update on this feature request? Not all environments support sticky sessions.
The bug is closed as design, with this comment:
"This is a known limitation. Changing it would be a new feature request that needs to be prioritized against other work."
I can hear you say "but this is a feature request" - I'm just the messenger. It means that adding the future is not a priority compared to the other work.