Even if you wonder what is happening the permissions on the file clearly state your "maas" user has read, write, execute permissions and nobody else; not even a group "maas" is part of.
For something to be problematic I would assume that the "others" part of the permissions (the last 3 dashes) would be "rwx".
Just looking at it from that perspective this seems perfectly normal to me.
If you look around the web the "maas-import-pxe-files" script seems to use /tmp/ to store temporary files. It seems more likely this file is from running that script.
Besides that "maas" uses "touch /tmp/block-poweroff" to not ask for a reboot.
So seems legit to me.