Not the thing where Marxist analysis predicts something that's obviously going to happen, people (libs) don't believe it and shout you down, then it happens 
Rather, it's supposedly how the spouse or partner of an autistic person endures some sort of suffering (usually in the form of emotional neglect) caused by the autistic person, but no one believes them since the autistic person masks around others.
To me it sounds like plain ableism, putting the blame of a bad relationship entirely on the autistic person for not perfectly conforming to the allistic's expectations. Honestly similar to "autism parents" who complain about their autistic child's existence and make it entirely about themselves and basically just straight up hate their child.
But I also understand that relationships absolutely can be one sided like that, even between allistics. Especially based on gender, i.e. the trope of the housewife doing everything to take care of the husband who does basically nothing after getting home from work. I can even see it in my parents' relationship; my dad - who is probably autistic, but also diagnosed with other stuff like bipolar depression - isn't great, and I strive to not be like him in many ways, although I haven't had a relationship yet.
That said, I still think on the balance the idea of and intent behind "Cassandra Syndrome" is just ableist.
Here are a couple of (IMO) good write-ups that I found, from the autistic point of view:
https://rationalwiki.org/wiki/Cassandra_affective_deprivation_disorder
http://abnormaldiversity.blogspot.com/2017/08/cassandra-syndrome-and-denial-of.html
It sounds a lot like untreated depression or a neurodivergent person being so traumatized into masking that they're afraid to be vulnerable even with those that they love, and in either case requires treatment, not being blamed for it. It would clearly cause suffering in the person they're emotionally neglecting, but shaming them for it isn't likely to do anything but make them withdraw further underneath the mask, maybe start bolting on some of the old standbys like abrasiveness and aggression to really push away the perceived emotional danger. Causing, you know, more harm and possible abuse on both sides.
A breakdown in communications will of course cause hurt feelings, relationship damage, even trauma. But I'm getting pretty tired of how therapy-speak is being co-opted by people to heavily stigmatize behaviors far and away beyond how problematic they actually are. ND people do in fact need to take stock of and get control of our feelings, and find effective ways to interact with the feelings of others. The alternative is an isolation so profound it pushes you to very dark places. But putting a name like "Cassandra syndrome" on self-destructive patterns of emotional shutdown and distancing behavior that people came to through poor social conditioning, trauma, etc. just dumps all the blame in their laps and becomes another barrier to healing.
I first heard it in a podcast, just yesterday.
I would like to learn more about this; I've certainly felt this way with past relationships...
