I'm getting the same behavior. Happens on Extraction for sure, and I believe I had it happen to me in Supremacy as well (though not 100% sure on that). I pick up the power-up, use it, and the selection screen comes up with one hero visible and the others grayed out. I can use my left/right arrows for about a second to move through the list, but no matter what I do it auto-chooses the one hero that was initially selected.
Playing Battlefront on PC/Origin.
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Now I'm wondering if this is simply by design, and that I'm mis-remembering it happening to me in Supremacy. Yesterday I played several rounds of Extraction on Jabba's Palace, and every time, Leia was the first hero to come out (during stage 1), followed by Han (stage 2). The Imperials seemed to get a choice for stage 3 though, as I saw both Greedo and Vader during different rounds.