MAAS 2.0 eliminates the differentiation between "temporary" and "static" DHCP addresses that was present in MAAS 1.9, so when running for MAAS 2.0, maniacs-setup merges the two ranges into one. This is all well and good, except that MAAS now has a separate "auto-assign" range (and has since 1.9). Under 2.0, I suspect that MAAS is scanning the local network and assigning anything that's NOT detected as in-use. Unfortunately, it gets it wrong sometimes -- I just had it assign the address that my laptop uses to a node. (Grumble, grumble.) If I'm right, it could also "auto-assign" an address that's in use by a system that happens to be powered off. Offhand, I don't know how to tell MAAS to use some other range for these "auto" addresses, but this should be investigated and, if possible, controlled.
I'm facing this at the minute as well. I get around it by deploying a DHCP server on the network and letting MAAS get an address within a range from that. Would love if it was possible to specify the ranges to be considered when 'auto-assigning' an address.