@Chris looking at this on a desktop now, your actual problem has to do with operator precedence. https://docs.oracle.com/cd/B19306_01/server.102/b14200/conditions001.htm#BABBCFGD.

AND

has precedence over

OR

so your condition would have been interpreted as

{memo} LIKE '%x%' OR {memo} LIKE '%y%' OR ({memo} LIKE '%z%' AND {postingperiod} = 'Jan 2019')

while you probably intended for

({memo} LIKE '%x%' OR {memo} LIKE '%y%' OR {memo} LIKE '%z%') AND {postingperiod} = 'Jan 2019'