yeah, I can't find what I read. so maybe I am wrong. I know I read something like : 'a player being called up after Aug 31st accrues a small amount of service time for that month, but it doesn't start his clock. '
however, since I can't find that again, I will assume it was a garbage or dated read.
I had that notion in my head as well until I started looking into Swansons service time when he got demoted.
I'm almost certain any time spent with the MLB team, even after rosters expand, fully count towards service time. The worst case scenario is when a guy gets hurt and spends early years of control on the DL, which is exactly what happened with Profar.
Having said that, if the plan is to have Acuna on the team opening day next year, there is no harm calling him up now. Hopefully they are smart enough not to have him on the opening day roster of a team that's shooting for .500.