That particular mission wasn't bad really, there were other, much worse missions,

in some you are also put in the middle of a mob of champion/elite mobs after the buff-removing cutscene. o.o

With the mission you mentioned, it aligned well with my buff cycle and I just had the CDs ready at the start of every wave and rebuffed.

But yes, by all means, this is whimsical, and STILL ...NO WORD from the devs about this! -.-

[edit:]
The thing that, if you want to get anywhere, using 4+ buffs is mandatory, is quite a bad sign for the class design of the psyker,
but that's a whole different story by itself.

I know how you feel, because i still remember with pain, the mission where you have to guard your tech guy while he jerks off in front of a destructible cover, (*derp) and as a psyker it was the most painful and frustrating experience i've ever had:

5 waves of enemies, EACH ONE followed by a "very much needed" cut-scene where my guys asks "you done yet?" And the thing is: EVERY TIME this occurs you lose all your active buffs, just to make you feel miserable for the next 60 seconds. Not to mention that everytime you make an "interesting encounter" with some anonymous boss or whatever he is, our 200IQ inquisitor rushes in front of his nose for the only purpose to bark some nonesense while his whole army of chaos marines are already laughing at us wating for the "very much needed" cutscene to end and start burning our health to 1/3 before we can even move one inch.

Bottom line: Cutscenes and gameplay SHOULD NOT interfere with each other like this. If you really care about adding this feature there is a method already tested and used for ages: 

-Screen slowly fades to black the moment you step into the boss room.

-Cut scene starts and ends.

-We get to control our character back at THE DOOR.

Whole rebels are way too overpowered they have too much super high dmg burst annoyance and hardest mob types compared to some of the other which are almost too easy.

I am THIS close to going to personally bug the devs about this until they give at least a response.

I know there are countless totally broken things since alpha, not only this, but it costs only a minute to say something about it.

... gotta laugh to keep from crying about it.

If you think that is annoying already, go make a psyker that lives and breathes by running 6 buffs at once, whereas 4 of those buffs have 60s cooldown and 1 has a whopping 2 minutes cooldown!

Buff are removed by:

- stuns of any kind, mostly dealt by (partly invisible) mines
- getting hit (or even barely touched) by several of the mobs charges animations
- cut scenes (seriously, neocore?!?)
- being on low suppression and then getting sneezed at by trash mobs

The most disturbing thing though, is how the devs have not lost a single word about this whole topic, ever!