MySQL UPDATE query partly updating with default values - mysql

Question

I am running an update query (in MySQL 5.6) on 2 tables joined like the following:
UPDATE c_cache cc
JOIN p_cache pc USING (user_id, attribute_id, calculation_quarter)
JOIN batch_table bt USING (user_id, attribute_id, calculation_quarter, client_id, group_code, version_id)
SET cc.epop = SUBSTRING(bt.result, 1, 1),
cc.excl = SUBSTRING(bt.result, 2, 1),
cc.num_result = SUBSTRING(bt.result, 3, 20),
cc.status = 'FR',
pc.epop = IF(bt.enrolled = 2, SUBSTRING(bt.result, 1, 1), 1),
pc.excl = IF(bt.enrolled = 2, SUBSTRING(bt.result, 2, 1), 0),
pc.num_result = IF(bt.enrolled = 2, SUBSTRING(bt.result, 3, 20), REPEAT('0', 20)),
pc.status = IF(pc.status = 'FL2', 'S', 'FR');
The p_cache is being updated properly, but the 3 columns in the c_cache is being set to NULLs and status to 'S' (all default values of these columns).
The rows that are being missed out are usually contiguous (in chunks).
This query is within a loop in a stored procedure that runs till all 'S' (stale) status rows of p_cache are marked 'FR' (fresh), i.e. computed.
(All rows of p_cache are present in c_cache with a one-to-one correspondence).
The batch_table picks up rows in batches of 25000 rows per iteration, and gets updated with computed results in result column through some stored functions.
This whole stored proc. is called from a MySQL event. Multiple events run simultaneously (each for an exclusive set of attributes) to find stale rows in the p_cache, and update both cache tables with computed results in batches using queries similar to this one.
This anomalous behavior happens only on the c_cache, but only sometimes.
The schema definitions are:
CREATE TABLE c_cache (
user_id INT(11) NOT NULL DEFAULT '0',
attribute_id INT(11) NOT NULL DEFAULT '0',
calculation_quarter DATE NOT NULL DEFAULT '0000-00-00',
version_id INT(11) NOT NULL DEFAULT '0',
epop TINYINT(1) DEFAULT NULL,
excl TINYINT(1) DEFAULT NULL,
num_result CHAR(20) DEFAULT NULL,
status ENUM('FR','S','FL1','FL2') NOT NULL DEFAULT 'S',
PRIMARY KEY (user_id, attribute_id, calculation_quarter, version_id)
) ENGINE=InnoDB DEFAULT CHARSET=latin1;
CREATE TABLE p_cache (
user_id INT(11) NOT NULL DEFAULT '0',
attribute_id INT(11) NOT NULL DEFAULT '0',
calculation_quarter DATE NOT NULL DEFAULT '0000-00-00',
client_id INT(11) NOT NULL DEFAULT '0',
group_code CHAR(5) NOT NULL DEFAULT '',
epop TINYINT(1) DEFAULT NULL,
excl TINYINT(1) DEFAULT NULL,
num_result CHAR(20) DEFAULT NULL,
status ENUM('FR','S','FL1','FL2','S1','S2') NOT NULL DEFAULT 'S',
PRIMARY KEY (user_id,attribute_id,calculation_quarter,client_id,group_code),
KEY date_status_id_index (calculation_quarter,status,attribute_id)
) ENGINE=InnoDB DEFAULT CHARSET=latin1;
Can anyone kindly explain why this is happening and suggest a way to avoid this?
Thanks in advance.