REGEXP_INSTR returns 1 when using brackets

MariaDB Documentation says about REGEXP_INSTR: Returns the position of the first occurrence of the regular expression pattern in the string subject, or 0 if pattern was not found.

My task is (simplified): finding o followed by a double consonant

SELECT REGEXP_INSTR('a_kollision', 'o([lm])\\1'); # -> expected 4 got 1

It returns 1 but I would have expected 4 as the position of oll

Doing the same with replace works:

# replace works as expected

SELECT REGEXP_REPLACE('a_kollision', 'o([lm])\\1', '???'); # -> a_k???ision -- OK

It seems that the trouble starts when using brackets in REGEXP_INSTR:

SELECT REGEXP_REPLACE('a_kollision', 'oll', '???'); # -> a_k???ision  -- OK

SELECT REGEXP_REPLACE('a_kollision', '(oll)', '???'); # -> a_k???ision  -- OK

SELECT REGEXP_INSTR('a_kollision', 'oll'); # -> 4  -- OK

SELECT REGEXP_INSTR('a_kollision', '(oll)'); # -> 1  -- wrong

Checked with latest stable version 10.2.6, default settings.