When a string, used as old_text for SUBSTITUTE(text, old_text, new_text, [instance_num]) function, is repeated more than once and the new_text is an empty string, one occurrence of the old_text remains not substituted.
=SUBSTITUTE(A1, "ab", "")
ab with another string, e.g.
cd, replaces all instances of
ab as expected.
113ab11 - when ab is repeated more than once in a row, one of its instances remains unchanged to an empty string