I read somewhere there requires a minimal version 31 of fastboot.
But the fastboot embeded in easy-installer snap is older :
/snap/easy-installer/32/easy-installer-linux-x64/bin/adb/fastboot --version
fastboot version 30.0.4-6686687
Installed as /snap/easy-installer/32/easy-installer-linux-x64/bin/adb/fastboot