-ate abilities don't change the Z-move's type as it would with regular Normal-type moves. For example, Pixilate Sylveon with a Normalium Z using Hyper Voice with its Z-power will result in a Breakneck Blitz, not Twinkle Tackle (originally, Hyper Voice is a Normal-type move, not Fairy-type).So Pixilate is said to change Normal type moves into Fairy type moves (obviously). Like a lot of people, I ran Hyper Voice on my Sylveon. I thought I could get a Z-Move off of it, so I gave it a Fairium Z. However, it was incompatible with the move. Then, I attempted to give it a Normalium Z. It was compatible, and Breakneck Blitz was listed as a Fairy type move because of Pixilate, so I used it on a Marowak-Alolan. However, it said it was unaffected, as if it was still a Normal type move. Then, I ran the Fairy type Breakneck Blitz against a Hydreigon (to confirm the moves type as Normal) and it wasn't super effective.
Perhaps this is how it is in the games (Breakneck Blitz retains the properties of Normal type, even with Pixilate), and in that case, please make sure to have Breakneck Blitz listed as Normal type when used on Sylveon.
If it's supposed to be ran with Fairium Z, please change that.
If it's supposed to be a Fairy type move, well then, please make it a Fairy type move.
I'm going to try to get a Pixilate Sylveon in SuMo to zero in on the problem (instead of having 3 different possible problems, not knowing which one is correct), and I'll repost my results if I do. Thank you!
In your case, Breakneck Blitz shouldn't be listed as a Fairy-type move (much like Hidden Power with the mon's Z-power can only be compatible with Normalium Z and will always result in a Breakneck Blitz, whatever typing HP is set to).
Hope this helps understand the mechanics of -ate abilities + Z-moves.