Sorry to hear about your loss, how are you doing right now?
Your post really spoke to me as I'm experiencing the loss of a grandparent at the moment, who would be considered a non-immediate relative. My grandpa was a very important person in my life, he was someone who for me was a sort of role model. I've felt that sense of disenfranchised grief that you mentioned at times. I've been attending therapy to talk through some of it.
With regard to your question about why some types of grief are not noteworthy, I've wondered the same thing. It seems to me that categorising grief is purely for the convenience of/ not disrupt or offend societal norms. Things like you must grieve for a particular amount of time, cry immediately at the sight of a departed loved one, only react with great emotion at the death of immediate relatives, keep emotions private, etc,. But, there really isn't one right way to grieve, or express emotion. Something that's tolerable for one person may be painful for another.
I have not known many people who seemed happy to comply by these norms. They mostly just did it to avoid judgement from 'others', these 'others' who probably themselves keep up appearances to avoid judgement. In my experience, adhering closely to such societal standards results in people consciously or subconsciously upholding me to such standards. Again, I could be mistaken in my view but this is what I've come up with so far.
This must be a difficult time for you right now but if there is anything you'd like to talk about with me please let me know.