Card unplayable timeout
I played a card just as my turn was about to timeout. The card approved on the stack but would not resolve. The card stayed on the stack for subsequent turns but was unplayable.

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Silvice#64315 commented
Yes, this just happened to me as well. This is similar to the "card stuck on stack" bug at https://feedback.wizards.com/forums/918667-mtg-arena-bugs-product-suggestions/suggestions/44110044-card-stuck-on-stac
However, for me I knew I was about to time out and I was deciding whether to play a card and I released it JUST as my main phase completely timed out including all timers and the game acted really weirdly. It placed the card on the stack but didn't cast the spell, and the life indicator on the card kept changing as I went though my end phase and subsequent turns. It stayed for the rest of the game before returning to my hand after I died. Logs and screenshots attached in case it helps anyone out.