Does it sound weird if we use sietnek instead of besietnek
No. It sounds more naturally to me. And sietnek is used more often.
Besietnek. I am Hungarian and in 26 years, Ive never heard anyone using "besietnek", but the braindead idiot who made these test obviously used google translate for 90% of the execises.
No one has ever used besietnek :D Sounds weird to me, but who knows... Anyhow, sietnek should be accepted as it means the same.