Can the revised beastmaster's animal companion gain the benefits of Rogue's expertise?

Question

As of 14 May 2017, the revised ranger has not been balanced for multiclassing. I'm looking for justifications, both balance and RAW, for allowing or banning this.

The Revised Ranger's beast companion "uses your proficiency bonus rather than its own"

The Rogue's Expertise class feature says that "Your proficiency bonus is doubled for any ability check you make that uses either of the chosen proficiencies."

What will this break, if anything? The worst thing I can think of is a +8 STR(Athletics) check on a grapple at level 5, but a Valor Bard can have the same bonus and Extra Attack at the same level.

Answer 1

It's not in the rules...

The rogue's Expertise feature says (emphasis mine)

Your proficiency bonus is doubled for any ability check you make that uses either of the chosen proficiencies.

The beast companion is not you, and when it uses your proficiency bonus (on its own skills, those it started with, and those you chose for it), you're not not using those skills, so the bonus is not doubled.

Why not do it anyway

If it were allowed, both the ranger/rogue and the companion would have the doubled proficiency bonus in the chosen skills at the same time.

The beast companion is another set of actions, mostly independent from the ranger, not just an extra attack. Adding in a doubled proficiency bonus at the same time as that would enhance the companion feature beyond its original scope, and beyond that of another character with doubled proficiency and extra attack.