IMO, It’s a simple bug.
I have Grave Digging in hand, it costs 1 because my Undead died after my last turn.
I use Core set Preparation, it supposed to make my next spell costs 2 less
My Grave Digging still costs 1, after Preparation was used.
I even played it, but it still costed me 1 mana to play.
Also I’d love to provide my screenshots, but don’t know where I can do it
You may have noticed that sharing any URL links will be disabled in some instances when replying to a post. There is a work around that will allow you to share links to screenshots, videos, and replays.
You can use either the Preformatted text option (the button with </>) or the Blockquotes button ("). It will display the link as shown in the examples below:
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Regarding your report - it may be intentional that the “Costs (1) if a friendly Undead died after your last turn” overrides all other cost increases and decreases. There have been similar interactions in the past.