I don't even know how to reproduce this error because it seems entirely random. I have untapped mana sources with enough mana available to cast an instant card in my hand or activate an ability of a permanent on my battlefield. But before I get the opportunity to do so, the turn passes to my opponent, or the phase passes during my opponent's turn. As in, I cannot act during my opponent's turn, and sometimes during my own turn, the game just decides that my turn is over when I still have other options that should be available.

  PC

  I have mana and an instant card or an activated ability of a permanent on my side of the battlefield. I do not click Auto Passing, I do not select Passing Turn, and I don't have any other effects preventing me from casting such as the effect of another card or lack of mana. However, during my opponent's turn, I seem unable to cast an instant spell or to activate an ability of a permanent that has an activated ability. It seems entirely random too. It happens without any explanation and is not tied to specific cards or game formats. It is also not due to inactivity timers since I am actively participating in the match.

 I would hope that casting an instant card and activating abilities at any time during the match, as long as they do not have "sorcery" timing rules, would be a regular and intended game mechanic and feature. Apparently not always and with no explanation.

  No screenshots or videos

  There is no workaround, and I have lost games because of this issue.