I'm having the exact same issue, can't tab between fields or set the tab order? i think the tab order is set by the hiarchy of the layers, but I can't tell...
I tested this a couple of months or longer back. My testing seemed to reveal the following behavior.
If all TEBs are in a vertical column, tab moved from top to bottom.
If perhaps you had two vertical columns, tab moved from top to bottom, but left to right as if you were reading a page in English.
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