It appears as if only plus codes of length 10 or 11 are currently
considered valid when entered in combination with a location name - while
length 8 codes are correctly interpreted only without location name. For
example:
*VALID*:
G98H+G3, Berlin, Germany
G98H+G3M, Berlin, Germany
9F4MG98H+
9F4
EDIT:
The whole of 9F4MG98H+ is located within Berlin *AND* the whole of Berlin
is fully contained within 9F4M+ (https://plus.codes/9F4M+) - in
combination, that's why the first four characters should be recoverable
from the string "Berlin, Germany". The same error exists for location
