MySQL LEFT JOIN with date filter
I checked around.  The bug seems to be in both MyISAM and InnoDB, MySQL 5.5 and 5.6.  It seems to be fixed in 5.7 and 8.0, plus MariaDB 10.

I did not find the bug report in order to identify what, exactly, was the cause, but here are some recommendations:


Don't use DEFAULT '0000-00-00'; "zero dates" are, by default disallowed in later versions.  (There is a switch to allow them.)
Don't use dates looking like 20200518, use instead '2020-05-18'
It may be that 20200518 works, but '20200518' fails (in 5.5 and 5.6).  That is, the quotes caused trouble.
Upgrade the version.
Reconsider the need for an AUTO_INCREMENT since you have a "natural" PRIMARY KEY (jma).