Yes, you found a bug Greg! Thanks or reporting this.
Same problem with September 2019. It is exactly the combination of the settings mentioned in your post both set to 1.
Easy fix: Edit the file views/month.php and change line 75
from . . .
$sOffset = date("w",$tfDay) - $set['weekStart']; //offset first day
to . . .
$sOffset = date("N",$tfDay) - $set['weekStart']; //offset first day (ISO)
Just the "w" should be replaced by an "N" in the date function (and ISO added to the comments at the end).
This fix will be included in the next calendar version, which I will release early 2019.
PS. I very much like the colors you have chosen for the different categories in the Church Diary; not too hard and not too soft, exactly right. I also read with interest your Updater's Guide, very clear explanation, I could not find any error