I face the same bug in standard leagues: if my opponent has casualties of war in their deck, the game becomes incredibly laggy for both players to the point where it's not really playable.

Blade of Selves reads:
" Blade of Selves {2}

Artifact — Equipment

Equipped creature has myriad. (Whenever it attacks, for each opponent other than defending player, you may create a token that’s a copy of that creature that’s tapped and attacking that player or a planeswalker they control. Exile the tokens at end of combat.)

Equip {4}
"

Issue:
When the Myriad tokens get created, those token copies don't get exiled at end of combat.

As far as I know, all other myriad cards have the same problem.

Opponent used Jace, Wielder of Mysteries and used its ability to draw a card and then it caused the player to win the game even though the player had 46 cards left in their library.

Permanents turned into elk by Oko after a Tabernacle of Pendrell Vale is in play do not actually gain "Pay 1 during your upkeep or destroy this creature."

My opponent controls Kazuul, Tyrant of the Cliffs. I attack with a creature, and resolve Kazuul's triggered ability, paying three for the trigger. I select 'yes' after tapping three mana. The ability resolves, and my opponent creates a token, despite the trigger having been paid for.

I am currently missing all sounds on MTGO except the start up music. Although all sounds are at max in the settings, and the audio folder is present in the file location. Also MTGO doesn't not seem to come up on my Volume Mixer so I cannot change it there either. I really need sounds to play the game. (I've just stopped Arena since the amount of bugs on there is too numerous to deal with now)

28/3/2020 my name is pietr
in my first challenge, game 2
After combat , did not go to main phase and gone right to end turn. Did not let me play a spell that would be very important to the game. Did not let me play “light up the stage”.
I am frustated. Thanks for the attention

When Lord of the Void deals combat damage to a player the game infinitely restarts

I cast Gideon's Intervention naming Uro, Titan of Nature's Wrath. My opponent attacked with that creature and damaged me twice. It is a bug that should be fixed but I shouldn't be penalized for that.

Opponent had a haktos with chosen number 3. I had an entrancing lyre. I activate lyre targeting hakdos which should work since the chosen number is 3. Usually when activating lyre it asks how much mana I want to spend, in this case it completly skipped that and didnt ask me. It just says I chose 0.

with typing [sW] i get white mana symbol. With [s...] how to type mana symbols such as {W/U} or {2/U} or phyrexian mana? This is not in instructions on the web site. On the web site some instructions don't work either. For example [sSpellA] for artifact symbol does not work.

When playing the 3 MOCS events today, I encountered this bug two times:
When playing two matches at the same time, I used "Yield Until here" in one game, and skipped until that point in another game. However, the first time it happened, I won the affected (Standard) match.
Later, the same thing happened again when I misclicked on the "Yield Until Next End Step" option in the Standard Showcase Qualifier, MTGO skipped my first turn in the Modern Showcase, and caused me to lose the Modern game&match.

The program didnt let me cast a force of will on a game ending crop rotation

Underworld Breach not allowing you to cast Zero mana spells such as crashing footfalls, ancestral visions, and restore balance. Text on cards read that cost was zero and required to exile 3 cards, but did not allow us to select the card to cast.

Opponent casts spell "Gifts Ungiven" and by only choosing 2 cards, I was unable to pick any cards to send to the graveyard. This is clearly not how the card mechanic reads. It states the casting player only received 2 cards which remain after opponent chooses 2 to go to the graveyard. It seems to be an exploit.

I click on "play with kicker" on Emblazoned Golem, I pay 2 mana and it comes into play with 2 mana in my pool, not allowing me to pay the kicker cost.

Steps to reproduce:
1. Begin casting Fire Covenant, choosing not to select any target.
2. Pay the mana cost of Fire Covenant.

Actual Result:
The game appears to hang, the prompt box prompts the user to pay one extra mana (whichever color was the last mana paid for Fire Covenant's mana cost). Performing any actions other than casting Fire Covenant again will result in a Game Server Reset. Casting Fire Covenant again will prompt the user to select any number of targets again.

Expected Result:
Fire Covenant is cast and goes to the graveyard. The user loses no life.

I played a baffling end to remove a Rhonas then used Calix's -3 to exile a Steel leaf Champion onto the baffling end. When he removed the Baffling End with a Natural state it returned the Steel leaf and gave him a 3/3 Dino as it should have, but also returned his Rhonas to the board as it shouldn't have. I would assume it had some interaction with Calix's return to the battlefield clause with the baffling ends exile effect.