tldr := Windows 10...
So I had Windows 8.1 / Linux dual boot system. And decided that, what the heck, I'll try upgrading to WinX. That went well. Surprisingly. I even used WinX for a while. Windows 10 and linux side by side, on the same disk, just different partitions. And since everything was so nice, I've decided to let Windows install some updates. After reboot, uefi linux entry is gone. What? How? Well, after update windows found 'unrecognized' partition, and, since it is urecognized, it must be unused. So it helpfully fucked it up. That was linux uefi partition (yes, I kept them separate).
Long story short, windows8.1 back from original image, side by side with linux. Lesson learned, gonna ispect each windows update twice before installing. And not gonna touch WX even with a 10-foot stick!