It seems Alphazero is deterministic, up to a point. Looking at the details of its implementation, there's nothing inherently random in it. If one looks at the TCEC bookless bonus between Leela (an Alphazero clone) against Stockfish, this particular Leela net always played the same opening move 1.e4 when it was white. Against this Stockfish sometimes played 1...e6 and sometimes 1...c5; Leela always responded with the Steinitz Boleslavsky variation and Najdorf English attack respectively. In fact, almost every time it's Stockfish deviating instead of Leela:
Leela is more stubborn than Stockfish in its opening moves. All the major opening classes in the first few moves - Sicilian, French, Ruy Lopez, Italian, QGD - were determined by Stockfish either in black or in white. Stockfish expanded the opening tree almost every move in at least one opening line, again in both white and black. Leela, on the other hand, almost always played the same move when presented with the same position. In the first 24 plys of 100 games I found only 3 cases where Leela expanded the opening tree, on move 7 as black in QGD (game 29), on move 10 as black in Ruy Lopez (games 45 vs 97), and on move 10 as white in French (game 84).
Although Leela was a lot less random than Stockfish, it wasn't completely deterministic either, presumably because of multithreading & random fluctuations causing a different number of nodes to be evaluated. Take out these factors, and Leela (and hence Alphazero) is deterministic.