The function row_sel_get_clust_rec_for_mysql() is invoking row_sel_sec_rec_is_for_clust_rec() even at the READ UNCOMMITTED isolation level, where we should just assume that a non-delete-marked secondary index record does exist.
One of the two calls is preceded by a condition that refers to the isolation level, but that condition seems wrong:
Because the lowest value for trx->isolation_level is
the condition <= actually is equivalent to ==.
I think that for READ UNCOMMITTED, we should only filter secondary index records by the delete-mark flag. If a record is delete-marked, pretend that it does not exist. This is exactly what we should already be doing for clustered index records.