Hi Sandstone,
I haven't got a clue on why they do this, I can only surmise/speculate.
Which I won't.
First I want to tell you:

for telling your current BF his drinking is a problem for you. You're setting a boundary: YOUR boundary. Excellent.
I've been a people pleaser for most of my life, and I have only started to learn about boundaries, but what is becoming increasingly clear to me, is that boundaries are personal. So everybody sets their own. So

for having set YOURS with regard to HIS drinking.
Getting invalidated by (supposedly) using my cPTSD as an excuse (or i my case: my FOO-dysfunctionality and childhood abuse. I haven't told anybody yet I identify as a cPTSD affected person) is a major stress-factor in my life as well. I'm witnessing friends falling away. Which adds additional hurt to the process of recovering from all this mess.
I'm sorry this is not painting a pretty picture, but I've come to understand this is, very unfortunately, part of the process.
There is a very encouraging (yet sad) video on this by a guy called "Narcissism Survivor" on YouTube:
Narcissism Abuse 4 Stages of Pain. A Trigger Warning should be added to that, as it's quite confrontational. He speaks specifically about what you describe as being part of 'stage 3', specifically after @8:15 in the video. (I think. At least it is what I relate to, both in his video, as well as with your post.)
With regard to me loosing friends, some of them teen-age friends, I try to find some comfort in knowing that it took me up to now to finally fathom the depth of the root of my cPTSD, so I can't realistically expect them to move any faster, or even keep up with me. Alas, that doesn't negate the fact I will have to leave them behind, if they can't keep up as I will (have to) go on.
Or perhaps they are leaving me 'behind' ?
Whatever it is, we are getting separated. And some of them are separating themselves from me, by putting the blame solely on me for the 'rift'. Which for sure isn't fair...
