That's actually the very definition of a plot hole.
"Gaps in a story where things happen without a logical reason."
It's actually not, you just think it is because you apparently don't know what the term "plot hole" means.
If you want to get technical, every mystery in a work qualifies as a plot hole until it's explained.
I'm sorry if you feel this means there's too many plot holes in entertainment, but it's not really up to you to define the term.
Another explanation might simply be that the bullet wouldn't make it through their Nen auras if they didn't suppress them.