Union glitch - Too many many parts?

[X_ZN0]

Hi everyone!
So to reduce some lag for my game with too many bricks, I wanted to unionize these stairs and to make it easier to create overall, I came around something weird, when I tried to create a union with the parts, it turned into some weird glitched mesh like union. Any way to fix it?
(MODEL WITHOUT UNION)

(MODEL WHEN UNIONIZED)

[Terrunda]

The reason why it is looking like that after unioning is because Studio realised the part count was high so it kind of glitched out.

Maybe you can try unioning fewer of the parts at a time and remain with a certain amount of unions? That’s my suggestion if you want to reduce lag.