The majority of the runs I’m not getting the cutscene and thus can’t fight the last boss. I’ve tried things like reloading and I can’t figure out if I can trigger it somehow.
Been reporting this bug for ages now, still not fixed 
Silly question but have you killed ALL bosses? Check your raid lock.
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And that you are doing it on Heroic difficulty.
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If it is already on heroic try killing Lei Shen a bit slower, sometimes bosses bug out if you kill them too quickly.
This is indeed the case for quite a few bosses, allthough i must say that this in itself is also worthy of some fixes
I’ve never seen that happen. Ra-den always is available after you kill lei shen.
So what i’m assuming is that you’re choosing the wrong difficulty, since Throne of thunder will have Ra-den ONLY on heroic difficulty and not normal.
Not showing the last boss for me
I have it set to mythic and 25, so it should automatically pick the highest one right? Last week I’ve done it without problems and I haven’t changed the setting in between.
I always try to delay it, actually got an achievement, so I guess I was slow enough.
The majority of my kills it doesn’t pop up.
Set the difficulty to HC and your choice of 10 or 25man, setting it to Mythic does not unlock Ra-Den as far as I know as Mythic was added in late SoO/WoD Pre-patch.
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you have to kill ALL bosses on heroic difficulty
It used to happen to me when I’d take the teleporter back too early. Stay on his platform a bit longer then the cut scene should commence.
This doesn’t make a difference afaik. When I enter in mythic, it will simply pick the highest available difficulty. Last week I entered on mythic and it worked fine.
That’s what I did.
I waited roughly a minute. Last week it took only a few seconds for the cutscene to appear.
Can you provide screenshot of your raid lockout?
This does not always happen.
I have gone into old raids that don’t have a Mythic mode with Mythic set, and got Normal. No, I am not mistaken. I have seen it more than once.
Look up your raid lock and double check what the difficulty says.
Someoneelse’s suggestion is good in general, but I’m pretty sure you literally can’t kill Lei Shen too fast because of the phases. Still, if you’re not seeing all the phases for some reason, it’s worth considering.
I experienced it more than once also, changing difficulty mid raid and swearing wtf.
https://i.imgur.com/1A80EB6.png
This is the only raid that gives me this issue I believe.
See screenshot above, it tells me 25, that’s all. I’m assuming it refused to go on Heroic this time? 
EDIT
It seems that more raids from MoP give me this issue…
Yep, you are locked to normal version of this raid. Next time remember to change difficulty manually.
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This difficulty system is constantly causing problems and confusion, wish they would fix it…
Yup, the screenshots shows that as Decayin says, you did it on Normal 25.
I’ve seen it on several. Most recently on ICC. Always check your difficulty as soon as you walk through the door.
I run ICC almost always twice a week with it set to mythic, always goes to heroic afaik 
Yeah, it’s not confined to specific dungeons, like Mists. It seems possible for it to happen on anything.