The value of sep is ignored while printing when two adjacent values are strings.

print("a", "b", "c", "d", "e", sep="-")

(base) saurabh-kumar@Awadh:~/Projects/System/lpython$ ./src/bin/lpython ./examples/example.py
abcde

The same is the case for integers.

print(1, 2, 3, 4, 5, sep=" is less than ")

(base) saurabh-kumar@Awadh:~/Projects/System/lpython$ ./src/bin/lpython ./examples/example.py
12345

Please note that this is not an issue with the keyword argument itself. When two adjacent objects are necessarily not strings or integers, the value is used.

print(1, 2, 3, 4, 5, "a", "b", "c", [1, 2, 3, 4], sep="->")

(base) saurabh-kumar@Awadh:~/Projects/System/lpython$ ./src/bin/lpython ./examples/example.py
12345abc->[1, 2, 3, 4]

The presence of a list leads to the usage of the separator.