This is not the first player this has happened. I have a SP with the Nationals and when I created him I game him 23 and he kept that number through the minors. When he got to the majors he still had that same number until late May then it was 24 then in early July it was changed again to 25. What's the story on that. In real baseball they don't change a players uniform number unless he asked for a change or a well known super player joins the team that wants that number. Neither is the case for my player.
It happens when another player has the same number. I assume the game gives it to the player with more seniority but it could also be based on rating.
I noticed it when I had an outfielder with the Twins. I chose #2. When I got to the majors, it switched to #4 because Arraez had that number and #3 was retired. When Arraez was traded that season my player got #2.
I also noticed when I had a player with the Rockies that Jose Berrios was #18 for three seasons with them and then Kris Bryant joined the team via trade mid season and Jose switched to #20.