We use NUM_VERSION for comparing versions. NUM_VERSION is defined as a simple string concatenation of the major, minor and release numbers, so it doesn't preserve version ordering:

• IPA 3.1.99: NUM_VERSION = 3199
• IPA 3.2.0: NUM_VERSION = 320

This causes an error when installing a replica:
ipa : ERROR A replica file from a newer release (3199) cannot be installed on an older version (312)

So far this only affects unreleased versions.