I sort of feel like he might be under orders from Mummy and is going 'is this supportive how fathers talk? Yes, this is totally how they talk'.

That actually reminds me a bit of Sherlock's relationship with John. "Not good?" "Bit not good."

Perhaps Mycroft didn't do too bad a job stepping in as Father. Or maybe it's just been a bad night for him.

I can also imagine once their father died, Trevelyan might have felt like the time for second guessing was over. One just had to keep calm and carry on.

Or Mycroft was totally an awesome oldest brother.