Single character C is excluded in RPLS

Summary:

In what case will Single character C get extracted out from a word and excluded from RPLS?

Content (please ensure you mask any confidential information):

When I am doing a RPLS health check for a client, it is noted that character C is excluded everywhere in the matching. For example, Country "CHINA" will be deemed as "HINA", Company "China Science Institute" was deemed as "hina S ien e Institute" in both UI and log. I checked both excluded words and property, and found no single "c" inside. Even if there was, "China" should never become "hina". Any pointer to the cause of this issue?