Ubuntu 21.04 is not loaded from the external drive ?

Greetings. I own a laptop Dell XPS 9560 and an external SSD drive. A couple of months ago I have installed Ubuntu 20.10 on the external drive. The external drive I used for work and native internal with Windows for personal purposes. As you all may know, a new version of Ubuntu has recently been released, so I decided to upgrade. First, I created a live USB stick. Then I disabled the internal drive (SATA) in BIOS to not interfere. Booted from the live USB stick and started the installation process. The existing external drive partitioning wasn't changed. I set the same root, home, swap, and boot volumes as they were before. The only thing is the root volume was marked to be formatted. The installation process was completed successfully. Then I proceeded with installing and configuring all the necessary software. The laptop was rebooted multiple times, nothing suspicious has been noticed. At the end, I enabled the internal drive and verified that Windows is still working fine. Now, when I'm trying to boot Ubuntu, I push F12 multiple times, select the external drive, then the Dell logo appears, but Windows is getting loaded instead. If I disable SATA, then nothing is getting loaded, the Dell logo hangs infinitely. Is something changed in Ubuntu 21.04 comparing to 20.10 that prevents the booting? Has anybody encountered a similar issue? Any idea how to fix it? Thank you. UPD: It appears to be a bug in Ubuntu 21.04. I have tried to install it on the‍ external SSD once again. Only this time I chose the option "Erase Disk and Install Ubuntu", which automatically creates the necessary boot EFI volume. Got the same problem. Then I have created a live USB stick with Ubuntu 20.04. Installed it using the same option. And it works ¯\_(ツ)_/¯