Shadow Priest: Bug or working as intended?

Playing my first priest I hit 60 not too long ago and basically lived in AV this weekend. First of all I love the shadow priest for pvp. This is my first character where rogues really aren't an issue, unless they outgear me by quite a bit. I don't raid, ever, so this weekend in AV is the first time I noticed a real problem with the shadow priest.
When fighting something with an entire raid I found myself almost entirely ineffective. Most of the time I just switched to healing. I am wondering if the problem is related to the 8 debuff limit. Here's what's happening:
Immediately after landing SW:P or Devouring Plague they disappear off the boss. They land (not resisted) but immediately disappear.
Mind Flay is similar. It doesn't get resisted (I have the 10% resist talent also). It simply stops before it even ticks once. Of course it still uses the full mana. I'm thinking it's because it has the snare component which is considered a debuff, and is therefore subject to the 8 debuff limit.
Consequently, I'm limited to only casting Mind Blast with it's long cooldown. I can't SW:P. I can't Devouring Plague. I can't Mind Flay. This completely neuters the class in this role.
Interestingly enough, both Vampiric Embrace and Shadow Vulnerability (Shadow Weaving) stay on the target with no issues.
So, what is going on here? This surely can't be intended.
When fighting something with an entire raid I found myself almost entirely ineffective. Most of the time I just switched to healing. I am wondering if the problem is related to the 8 debuff limit. Here's what's happening:
Immediately after landing SW:P or Devouring Plague they disappear off the boss. They land (not resisted) but immediately disappear.
Mind Flay is similar. It doesn't get resisted (I have the 10% resist talent also). It simply stops before it even ticks once. Of course it still uses the full mana. I'm thinking it's because it has the snare component which is considered a debuff, and is therefore subject to the 8 debuff limit.
Consequently, I'm limited to only casting Mind Blast with it's long cooldown. I can't SW:P. I can't Devouring Plague. I can't Mind Flay. This completely neuters the class in this role.
Interestingly enough, both Vampiric Embrace and Shadow Vulnerability (Shadow Weaving) stay on the target with no issues.
So, what is going on here? This surely can't be intended.