D&D 5E Robot’s Best Friend

So I have a question about the Steel Defender stat block. Do you add your proficiency bonus at third level to the stats of Might of Master trait, or does that only happen when you gain a bonus in proficiency? Example at 5th level the stats increase. The player of Stahl The Miniforged, asked this question since it isn’t perfectly clear with it having a +4 to Perception. (We don’t know if it has expertise or something which is what brought up the question.)

The following numbers increase by 1 when your proficiency bonus increases by 1

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Your proficiency bonus at level 3 is +2. It has not increased yet, and will not do so until level 5. Furthermore, you are not modifying the Steel Defender's Proficiency bonus when you change this number, it is a different named bonus.

As a side note, it's best to homebrew edit that particular sentence into:

"The following numbers gain a bonus to rolls equal to your proficiency bonus minus 2"

It prevents problems with multiclassing, and magic items or effects that would somehow change your proficiency bonus (such as Ioun Stones).