Oracle expdp command executed in a command file not honoring %U instead using _U without a sequence
You can escape the % sign to stop Windows swallowing it before Oracle gets a chance to see it:

... dumpfile=TEST_20190906_%%U.dmp ...


As a quick test, in a .cmd file:

@echo expdp ... dumpfile=TEST_20190906_%U.dmp logfile=TEST_20190906_expdp.log
@echo expdp ... dumpfile=TEST_20190906_%%U.dmp logfile=TEST_20190906_expdp.log


then running that from a command prompt shows:

expdp ... dumpfile=TEST_20190906_U.dmp logfile=TEST_20190906_expdp.log
expdp ... dumpfile=TEST_20190906_%U.dmp logfile=TEST_20190906_expdp.log