Could you elaborate on what you did, what you expected to happen and what actually happened? I've just tested it on 65.0b7, and there doesn't seem to be a problem as far as I can tell. (It may either be a bug in the code or simply an issue with the lack of documentation.) Thanks in advance!

Reply: Thanks for your response! Did you set the transition rule via "Edit Defaults" or "Edit Transition Rules"? In your scenario, you should use "Edit Defaults". "Edit Transition Rules" edits the transition rules for the currently loaded URL, so if you for instance do that while in the gmail tab, it will be functionally telling it "if you open gmail from the Google container, then open it in the Default container instead". (This will also not do anything if you just follow links around gmail, because "navigating around the same website should not change containers" takes precedence.)
(I understand that "Edit Transition Rules" may be a misleading label for the button, but "Edit Transition Rules for this URL" is too long...)

Reply2: Right, thanks for trying it out and sticking around to figure out the issue! I added an attempt at a workflow illustration to the addon description, but I probably should come up with better positioning and/or labels for those buttons too.