Please see the pinned post in the Unearthed Arcana forum, specifically the introduction:

All three of these subclasses use a new mechanic that is introduced in this playtest cycle - subclass features that can be used a number of times equal to the character's proficiency bonus before a short/long rest.

The best solution we have right now, without undertaking development work, is to scale the number of uses off the character's levels in the class to which the subclass belongs (e.g. Rogue for the Phantom).

What this means is that the number of uses displayed on some subclass features will be correct for single-class characters, however for multiclass characters, they may well see completely incorrect numbers of uses.

Obviously this isn't ideal, but we believe it's a better implementation than leaving these subclass features without the ability to track limited uses.