Bitter Triumph
The card Bitter Triumph seems to work in ARENA different as in the past. It says pay 3 life or discard 1 card. The first works but it is not possible to discard.
Urza-111#64879
shared this idea
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bonsai#70698
commented
I wonder if this is an old bug resurfacing post-patch. Bitter Triumph had a bug shortly after its block release where it would get "stuck" on whatever initial casting cost was used such that no alternative casting cost was possible for the remainder of the match.
I posted on this previously https://feedback.wizards.com/forums/940206-mtg-arena-archive/suggestions/47521073-you-cannot-pay-with-life-when-casting-bitter-triu and it was fixed, but maybe there's been an unintentional reversion.
FWIW, one consistent source of individual card bugs has been alternative casting costs.
Most recently, it occurred with Rottenmouth Viper https://feedback.wizards.com/forums/918667-mtg-arena-bugs-product-suggestions/suggestions/48955952-new-rottenmouth-viper-mana-discount-calculation-b