After several weeks of testing ultimo4k image vs duo2 image I have two issues I have to share with.
Both actually related to USB sticks. I tested 3 different ones with different partition formats. While all of them works fine on duo2, none of them is properly mounted on ultimo4k after reboot. I used to keep some cam log/config files, picons and/or swap.
Unfortunatelly, I had to move cam log/config files to flash due to cam never starts after reboot while files are located on usb stick, eventhough usb is mounted. That is why I assume, cam script is called out prior to mounting of usb stick.

Second issue is related to the first one and it is swap file located on usb stick. Swap is never activated after reboot. I have to do it manually over telnet/script/VTi menu.
If I add another issue to above which are picons not displaying on infobar after reboot until channel is switched you can simply assume that something is wrong with usb mount stage, at least here.

Non of these problems occur on duo2 image.

Did anyone of you guys notice something similar?

Where exactly did you see an option "automatically mount known devices"? I do not remember seeing such option even on duo2. I have never had to use it on duo2 if really exists somewhere. Moreover, usb stick is mounted after reboot - I suspect mount stage is started to early comparing to start of other stuff located on usb.

I do not want to start discussion about swap is necessary or not. Just looking for explanation of such behaviour.

The most annoying problem is epg.dat located on usb. It is also NOT imported after reboot, so after I have to restart GUI again.

Thanks, found it and will test it but do not think this can fix that as I already said usb is mounted. I checked all logs as well and nothing wrong in there. I give up for now, no more ideas where to look. I flashed everything from scratch 2 days ago. Do you own ultimo4k and use it along with usb stick?

I mentioned in OP that on duo2 everything works fine.

Thanks for reply. You might be right saying box is too fast, I did not think about it that way, but ...
I just bought new SanDisk Cruzer Ultra USB 3.0 16GB 80MB/s (SDCZ48-016G-U46) for test. The same situation. Config files and picons are not the problem, I have already moved them to flash. Swap is also not a problem, I can live without it. Log files located on usb and epg.dat are the problem. I do not want to place cam log files in flash that's why usb stick is necessary. If I do reboot, cam logs are not started for some reason so I have to restart cam manually to force cam starting to write logs on usb.. Everything which rely on usb stick does not work properly after reboot and I do not know why ultimo4k behaves in such way.
If this is true what you suggest, looks like USB ports are useless in this box. I will have to live without usb stick. Will test it under different image...

I did some additional tests and found solution for myself at least. I created script in rcS.d and placed it just before devicemanager script is started. Inside the script there is only one command to mount my USB stick by-UUID instead of mounting by non-persistent device naming like devicemanager does it and guess what. All works fine now, 5 restarts without mounting problem, USB stick is properly mounted along with swap and logs activated. This somehow proves that the way devicemanager mounts USB sticks is not efficient or fast enough for ultimo4k or there is something else which breaks mount stage during devicemanager tasks but I did not find any errors in logs.