From Jenny:

XNAT used to use the node number in its prefix for assigning experiment and subject ids, eg. CNDA07_E#####. It’s not doing that anymore since the 1.8.1 upgrade and we’ve had multiple id collisions between the nodes.

EDIT: it seems like the missing key here was that it would affect systems with 8+ nodes. The host entries with IDs 8 and 9 would get coerced to 08 and 09 which would then get interpreted as "octal digit 8" and "octal digit 9", which don't exist.