Hey Matsku,
Thanks for the report, but I can't seem to reproduce this on my own in a local server. I've also never noticed anything this bad in Multiplayer.
When you 'strafe' left the post indeed disappears behind the notch in the sight. However, after about half a second, it comes back.
It's maybe a little un-even from when you 'strafe' right, but not too bad imo.
Overall the 'breathing/ swaying' animation when aiming down sights is a bit slower than others. This is why it takes a little longer for the sight to 'cycle' back to its normal position. The Mosin Nagant actually seems to have the same 'breathing/sway' animation. The Nagant just seems to have a better sight picture so the post never disappears behind the notch.
The devs could maybe tweak it, but imo it has more to do with the M/39 having a worse sight picture than the Mosin Nagant.