I've got Ubuntu 18.04 and Windows 10 running on my home PC, dual-boot setup with grub asking me which to choose at startup. Ran the usual updates on Ubuntu yesterday (not sure what was updated, though it was a hefty one). The update did ask whether I wanted to overwrite my edited grub file (I had previously increased the amount of seconds before it loaded Ubuntu, the default) with a newer version and I declined.

Well apparently that was the wrong choice because now the PC goes straight to Windows 10.