Informix index names must begin with a letter. For reasons that elude me, they decided to create "unnamed" indexes (such as those that support constraints) in the format " NNN_XX", i.e. a space, followed by systables.tabid, followed by an incremental number.
However, when liquibase records these indexes in the changelog, the space in front of the number is lost.
How to fix this:
If you're generating a CREATE INDEX statement and the index name starts with a number, prepend a space and wrap in quotes.
Informix 12.10.FC9DE, but should repro on just about every version of Informix