IIB sometimes reads garbled text from an Oracle CLOB

Question

I have an IIBv10 (fix pack No 19) application which reads a CLOB field from an Oracle (v12) database and stores into a shared CHARACTER variable. I also write the variable content into a log. It usually works perfectly but sometimes in some environments I see that the text from the variable is different from the database text (e. g., one character might be different). What can be the reason for that? It seems to be reproducing sporadically, both in a multi- and a single-instance environment.

Sample code (MY_TABLE has a CLOB field and MY)

DECLARE MY_CACHE SHARED ROW;
DECLARE mySelectStatement CHARACTER 'SELECT * FROM  MY_TABLE';
SET MY_CACHE.Item[] = PASSTHRU(mySelectStatement);