He's reportedly complaining of 'double standards'. The only 'double standard' I can think of is perhaps Antony. But there must be a reason he's picked Antony over Sancho. Obviously, we're not at the training ground so we can't say for sure but from the outside, even though I'm the first to criticise Antony for his performances, one thing I will give him is he does put in a shift. I can see why from a tactical POV Ten Hag would want him over Sancho.
All I can say is, from the outside, it's unprofessional from Sancho. It especially does not look good on him when he was given time off randomly last year for 'unknown' reasons. Ten Hag has defended him in the past as well I'm pretty sure. If reports are true that he's not willing to back down and claims there's a double standards issue, then fine, just move him on. But I don't think he's gonna be very successful if that is the mindset he has. I could be wrong but complaining and being petulant like this usually doesn't pan out well in the future.