The following numbers increase by 1 when your proficiency bonus increases by 1: the homunculus's skill and saving throw bonuses (above) and the bonuses to hit and damage of its attack (below).
However, this phrasing is pretty vague now that it isn't a core class trait. When it was a class feature of the UA artificer's Alchemist subclass, it was given to you as an Alchemist at level 3. So I had assumed that meant that it would get +1 to all the traits from that point on whenever you increased your proficiency modifier.
However, it isn't simply given to every artificer with access to it anymore. It's an infusion you can choose any time you choose an infusion starting at level 6. So... does your homunculus:
get your proficiency bonus as a whole added to its stats (so +3 to everything at level 6, increased to +4 at 9, +5 at 13 and +6 at 17)?
get +1 to its stats every time your proficiency bonus increases from the point when you do make it? (so if you made it at level 17, it never gets any better than its base stats because your proficiency bonus never increases from then on)?
get +1 to its stats every time your proficiency bonus increases starting from the first time you could make it (level 6 on)?
get +1 to its stats every time your proficiency bonus increases from the starting bonus of +2, like I had assumed when it was still an automatic feature of the subclass (so it would get +1 at level 5, +2 at 9 etc)?