Something is wrong with the file names and hashes on the download pages to the point where it is impossible to know whether a downloaded file is the correct one.
At Raspberry Pi 64-bit selection you get two choices:
18.04.0 and 20.04
File and hashes are listed as:
ubuntu-mate-18.04.2-beta1-desktop-arm64+raspi3-ext4.img.x
233c268963637c94a3527b90fce3cf74eeaaf03e9d3a5623457b4b2412778daa
and:
ubuntu-mate-20.04-beta1-desktop-arm64+raspi.img.xz
82019e65bf836c10a7f4f787757e6f317ea8b9508f15ee23d25a85ef5107e9bc
The files and hashes you get when downoaded (Many tries!!):
ubuntu-mate-18.04.2-beta1-desktop-arm64+raspi3-ext4.img.xz
BB4F6F722B41AA4C40B710B3CB81936127ECF799236CCBAA3B6B5455D8BF2
B39
ubuntu-mate-20.04-beta1-desktop-arm64+raspi.img.xz
233c268963637c94a3527b90fce3cf74eeaaf03e9d3a5623457b4b2412778daa
Furthermore, for 32-bit 20.04 you have this correct combo listed with a duplicate hash from one of the other files:
ubuntu-mate-20.04-beta1-desktop-armhf+raspi.img.xz
82019e65bf836c10a7f4f787757e6f317ea8b9508f15ee23d25a85ef5107e9bc
That last one is the only correct combination.
Downloading the 64-bit RPi image (233...) and running on an 8GB Rpi4 gets a crash early in initialization but I can't really even report a bug since I have NO IDEA if I am using a valid file.