If you gain minor illusion from Shadow Arts, Improved Minor Illusion from Wizard doesn't let you select an additional cantrip.

To reproduce:  Pick monk up and level 3, pick shadow monk for monastic tradition.  Then pick wizard and level 2, pick school of illusion for Arcane Tradition.  Check your number of cantrips.  They say 3.  Should say 4.

+1 for this bug.  Created a lvl 1 Wizard with Minor Illusion as one of my starting cantrips.  After selecting the Illusion school for my tradition the additional cantrip that should be available to me is not.  If I rollback the level, choose something other than minor illusion, and level up again choosing Illusion school minor illusion is added to the cantrips as expected.

Shadow Arts doesn't need to be selected for this bug to occur. I made a straight-up Forest Gnome Wizard, chose Illusionist as my arcane tradition, and I'm still limited to three cantrips. I'm assuming this is because the Minor Illusion cantrip comes from my racial ability, and whatever check runs for whether the character already has Minor Illusion doesn't factor it in if the cantrip is a racial ability.