Yes, I found the same issue on Manjaro i3 ISO (21.0.2 at the moment). The good fellas at Manjaro’s need to fix this one in the next releases. Memtest is still there but the grub’s menu is unable to invoke it because the variable inside it ($1) becomes “Memory” instead of “memtest”.
In the meantime you can easily use memtest by selecting “Memory Test” and pressing letter e (Edit). Then, change the second line from linux16 /boot/$1 to linux16 /boot/memtest and press Ctrl-x to boot.