Similarly to T162774, log_search access broke when on some servers ls_field_val was converted into a PRIMARY KEY. Mediawiki still defines the index as secondary. In order for the query to work, the FORCE INDEX (ls_field_val) was converted into IGNORE INDEX (ls_log_id). https://gerrit.wikimedia.org/r/351653
Maybe the hint is not needed anymore or maybe it has to be converted back into FORCE INDEX (PRIMARY). Evaluate its impact and see if the index hint is still needed, ans followup on changes on mediawiki.