My POV? Maybe you feel like you intruded on an intimate moment. It does not sound like you were asked if you wanted to watch that go down. It sounds like you just walked in on this already happening by phone, and you didn't know if you should stay/leave or what. You were not prepared, and now he's wanting comfort/cuddly stuff you were not necessarily prepared to give.
Maybe could be cranky with BF about putting you in awkward space. So maybe you feel trespassed upon, and in the confusion, maybe hard to articulate because
HEY.... here's this "bigger" break up thing going on... if anything SHE could be feeling trespassed upon so what's YOUR problem? Peanuts! ANd he's grieving here so what's my problems compared to that?
Are your thoughts running like that? Swirling around trying to get surer footing? Or trying to remove you feeling bad in an unsatisfactory way like " I have no right to feel bad here because they must feel worse!"
Could try to not assign value/judgement to it. Don't logic that part of it. Just feel whatever it is you are feeling and let it blow on through. Emotional internal weather is just that -- emotional weather.
I will tell you what your problems are compared to that. They are YOUR problems. Maybe not as VIVID as whatever she has on her plate or what he has on his plate. But still equally VALID. Your plate is your
plate. BF could not be piling extra on your plate without your knowing/permission/willing/checking in first.
You still could be treated well in your own right. Their break up scene doesn't negate the fact you could have been trespassed upon by BF.
Could let a day or so go by and ask BF to not do things like that to you in future -- esp if you plan to polyship and both of you see other partners. Could give it a pass for him because he was emotional, needs to learn hinge skills, etc. Could give you a pass because you were "deer in the headlights." Could apply logic in this area -- the what to do next? -- portion of the process. Not the feeling part of the process.
It seems it was the first time either of you dealt with "breaking up" scenes together as a couple. To move it to better spaces in future?
Could take the opportunity to talk now then. Could talk about how you want to break up with other partners you each might date (should it come to pass) and how you want to break up with each other (should that come to pass) and get his preferences. So you do not experience something like that again.
I know it is not all of the onion layers going on here right now, but it could be one of the layers that's causing you some stink?
If you don't feel ok in yourself because you were confused and trespassed upon you could assert yourself to clear the air and clarify/clear up the boundaries.
Then see if you feel better after doing that.