Pact of Negation ability doesn't allow activated abilities to respond by default
If you cast Pact of Negation and you don't have the mana to pay the "you lose" tax, BUT you do have an activated ability that will win you the game during your upkeep, Arena will by default not allow you to use the activated ability unless you lock in priority control on your opponent's turn.
For example, you play Goblin Charbelcher. Your opponent casts an artifact removal spell on their turn, you counter it with Pact of Negation. You have 4 red lands in play. The turn passes to you, and you should be able to activate Charbelcher during your upkeep because the Pact of Negation "you lose" ability is on the stack to respond to. But instead, Arena's UI will cheat you out of a win.
In order to make it work, you have to CTRL, then Shift + CTRL before you cast Pact of Negation to lock in full priority control, so you have a chance to use the activated ability in response.
The UI by default should allow you to respond to anything on the stack if you have an instant speed action that can be used, regardless of what phase it is.
Same here, even in some cases for any reason arena won't let me activate charbelcher even tho no Karn or pithing needle or anything like it on the battlefield.
I have also run into this bug. I set a stop on my upkeep (while it was my opponent's turn), but Arena had me lose the game before hitting the stop. The trigger should come up with a "Resolve" button. (Note that I was playing on mobile / iOS, in case that is relevant)
I also had this just happen with Charbelcher during my upkeep. Tried to activate Charbelcher in response to the delayed Pact trigger and lost the game instead.
This same issue just happened to me. I had the mana to activate the Charbelcher during my upkeep. But as soon as the game turned to my turn, it just told me I lost the game. No stack. Do not pass Go. Do not collect $200. This cost me the game.
You can set a stop in your upkeep step and you will be able to respond with Pact trigger on the stack without relying on full control.