The answer is case sensitivity.
This answer was in fact provided by mustaccio in a comment, but they don’t seem to want to add it as an answer. If you see this, @mustaccio, please add your own answer, so I can accept it.
Anyway, it turns out that WAS appears to effectively put quotes round the values you specify in the “Custom Properties” section. I had entered M51Dev for the currentSchema property, and it was being sent to DB2 as "M51Dev". But DB2 had stored the schema name as M51DEV It sees those as two different values, of course..
Specifiying M51DEV for the property value fixed the problem.