Distribution statistics on VARCHAR / VARGRAPHIC columns?
It is "partially documented"/"not really documented" AFAIK. More specifically, there is APAR:

IT13369: SUB-OPTIMAL QUERY PERFORMANCE WHEN DISTRIBUTION STATS COLLECTED ON STRING COLUMN WITH A COMMON PREFIX LARGER THAN 32 BYTES

that mentions it. I assume what you actually care about are cardinality estimates for a query that has predicate on a column that has a common prefix in which case you might try the workaround from it (i.e. NOT collect distribution stats for that column)