While not exactly a bug, it's a pretty funny occurrence.
I cast Imbue Champion when I had 3 essence left on the sovereign.
Of course, that brought the essence down to zero. The sovereign was no channeler any more.
And non-channelers cannot increase essence on level-up, either.
And the only channeler who could re-Imbue the sovereign would be the soverign herself.
Maybe imbue should only work if the sovereign has > 3 essence...