I’m wondering if it might have been longer ago than a few weks if you are dealing with a 6 month period as per your shortcut. Birthdays are dates in the past so they are strictly speaking never upcoming … but I think Shortcuts is trying to interpret the request around anniversary … up to the end of the anniversary year. That last part I think is the root cause of your issue and why the custom shortcut is having issues returning to what you expect; and I agree it is a little frustrating if that is the case.
I’ve had a go at quickly throwing together something that I think will work based on my above assumption (based on a few checks, but I could be wrong about all this). Take a look and see if it is something you could adapt to work exactly how you want.
Can you elaborate? What is “it” and where are you seeing the year as 1604? I don’t have any examples that are coming from my contacts or the shortcut that I see matching year.
If I know where you are seeing this and the source, I can maybe have a stab at they why and possibly even addressing it.