I figured out what was causing the problem and I'll post it here in case anyone else has the same issue.
I was setting this particular UIButton's isEnabled state to 'false' in my gameManager's Awake() method if the player had not previously unlocked a corresponding level in the game. Once I placed this code (which checks the player's PlayerPrefs to see if they had already unlocked the level, etc, and then sets the isEnabled state of the button) in my gameManager's Start() method everything worked just fine again.
Using the Awake() method was working fine prior to 2.3.4, so I'm guessing something minor changed in the how/when of UIButton's code for storing the enabled color of a button before setting isEnabled to false.
I probably should have been using Start(), anyway. It takes awhile learning Unity to really figure out when to use Awake(), Start(), OnEnable(), etc...