The following type of query is using where in EXPLAIN instead of index for some large quantity of OR's (i.e. on 10.5.9 uses WHERE, on 10.3.x uses INDEX also on 10.5.9 takes many minutes, on 10.3.x takes seconds)
query: SELECT col2 FROM mytable WHERE col1=a OR's col1=b ... with lots of OR's
If we force the query to use the index, "FORCE INDEX(mytable_col1_idx')", then the 10.5.9 query will finish in time similar to 10.3.x
See attached files to reproduce.