I agree with
@Piggies&buns .
Guinea pigs have a much, much finer sense as to spotting (most likely smelling) illness than we humans. Compared to other species we are quite frankly pretty head-blind. Bramble would have known long before you that something wasn't right and that his mate was dying. He will have done his grieving before.
Perhaps my own experience with my dad's three years long battle with terminal cancer can help you put it into perspective. You can put your own life on hold for a few month but not for that long. My dad was several times very close to dying, depending on whether a new treatment had worked or not. I even married in the fastest time possible to fulfill my dad's dearest wish for me to see me happy and safely wed - not knowing whether he would be still around in 3 weeks (thankfully therapy #4 did work and bought him another year).
When he finally passed away, the overwhelming feeling was a sense of relief and closure because we all had run the full gamut of all the strong grieving emotions not just once but several times and wanted him mainly to be free of his suffering, especially the enormous pain attacks. We all had had our time with him to say goodbye, had been able to say everything we needed to sy to each other, and we had all done our best to give my dad what we wanted most from us for his own peace of mind to help ease his own journey.
It's not lack of love or lack of bonding but the fact that in a terminally ill one death can come somewhat towards the end of the acute grieving process and is not the start of it, as it is for you. Just a very different kind of journey. Overall, you do not grieve any less, just the timing and the dynamics are very different. Even as a human, you can never tell how you'll react to the death of a beloved one, depending on the bond, the circumstances and how much of the grieving homework you have already done.
Please take comfort from the fact that Bramble is holding up so you have time to get over the worst of your own acute grieving without having to feel guilty about him right now. That is a huge burden off you. Look after yourself in the coming month since you have been given that big boon.
In the long term, Bramble will however need the daily stimulation and companionship from a mate either directly or from a neighbour through the bars.
Our singles care guide contains a practical chapter about how to spot the different ways it can come out when Bramble's species needs for companionship and round the clock stimulation are not covered by you as the owner:
Single Guinea Pigs - Challenges and Responsibilities
Looking After a Bereaved Guinea Pig