Virgin new out-of-the-box RB5009 shows bad-blocks: 0.1% in system info. Same on 7.12, 7.11 and 7.8.
Is this a kind of software rounding problem that everyone has or it is only me is that lucky? 
It’s not a problem at all. Seems like netinstall makes it go away in my experience but it isn’t a problem regardless.
Thank you, I’m not sure if I can run netinstall from MacOS. But will definitely try when I’ll have spare PC.
Honestly, it’s not even worth the effort. It has no impact on router performance and is not indicative of any issue at all.
Thank you! Netinstall didn’t helped. 
Thread on the same issue
http://forum.mikrotik.com/t/hap-ax3-bad-blocks-0-7-i-brought-it-in-may-2023/168400/4
Netinstall formats flash disk in a sense of writing new filesystem metadata. But I highly doubt that it does low-level format of flash which would re-evaluate status of memory cells (marked as bad previously). And even if it did do that, the result might be the same state (bad memory cell). This might even be due to manufacturing deffect.
Whether such an error on brand new device is enough to trigger warranty is a separate (legal) topic … as in: how much of flash capacity can be missing to proclaim whole device as deffective? Or: how much of flash capacity can be missing to proclaim that functionality of device is significantly degraded?