Replying to this old thread because I had the same problem this week on the Tinker Board (not S) R2.0:
I tried both of the Android 11 images with the same issue as @AJofre - after running adb reboot-bootloader the logo screen appears but the device gets stuck on that screen and the device is not visible when I run fastboot devices
Tinker Board R2.0 /Tinker Board S R2.0 Android 11 V1.0.6
Tinker Board S R2.0 Android 11 V1.0.0
However, I was able to work around the problem as follows:
Reboot the device to boot normally (I had to power off the Tinker Board, remove the SD card, power it on again, power off, reinsert SD card, power on again)
adb shell
su
dd if=/storage/emulated/0/Download/magisk_patched-XXX.img of=/dev/block/by-name/boot
adb reboot
After the reboot, I'm able to grant root permissions to apps that request them.
