Informix foreign key constraint lost

Question

Informix - Foreign key constraints are dropped automatically when the reference table is altered.

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There are two tables - table_main & table_reference. A column in table_main, say reference_id has foreign key constraint to a table_reference.id. These tables are created using liquibase changesets and the foreign key constraint is successfully created during table creation part. But in future, more changesets are added to table_reference table, that alters the table schema as well. Finally after all changesets are run, when we check the constraints list on table_main, the foreign key constraint is missing. This issue is observed on Informix DB. Any idea on this, folks?