Well, this isn’t even a particular talking point for this genre. In general I always considered difficulty spikes (in any genre) to be bad. Level 7 should be in difficulty somewhat around level 6 and level 8. Bosses are ‘allowed’ to be slightly more difficult, but it shouldn’t suppose a big spike as I said.
Let’s explain this with number of tries to beat something as proxy for difficulty. If level 6 and 7 is a normal level which needed 2 tries to beat it each, and level 8 is a boss, I expect it to beat it with 4-5 tries. I shouldn’t need 15 tries to beat it.
I guess this is potentially worse in roguelites, as you won’t have a autosave point just before the boss to try it again directly.