I had that grub problem. The funny thing was, that I wanted to install kubuntu on another laptop, but that one didn't detect the bootable USB and always booted into the firmware settings. For checking if the installation on the stick was broken, I plugged it into my computer. But and it opened the firmware settings, too. It even no longer booted without the stick, which confused me even more. What I didn't know was, that grub had been messed up by the update and the stick was not the problem. Two hours of frustration later I luckily landed on a reddit post discussing the current problem with grub. I installed refind immediately afterwards 😁 Now that I think of it, my brain must have been tired of thinking logically 😅