MySQL update returns 0 rows affected in transaction

Question

Table account have special field 'executor' for daemons to know that account is blocked for update with one of daemons.

Step by step:

1. start transaction

2. block account with:

   UPDATE `account` SET `executor` = 2727 WHERE `executor` is NULL AND `id` = 1234;
   

3. some operations. During this step another daemons can select row with id=1234 and try to update it(or update another fields, not 'executor')

4. account status back with:

   UPDATE `account` SET `executor` = NULL WHERE `executor` = 2727 AND `id` = 1234;
   

5. commit

Some times at step 4 mysql returns success execution status, but 0 rows affected. It happens 1 time at 10000 transactions.

What am I doing wrong? Or its mysql bug?

UPDATE:

after the observations I get the following:

1) first we do

UPDATE `account` SET `executor` = 2727 WHERE `executor` is NULL AND `id` = 1234;

2) transaction blocks record

3) during transaction I do

`UPDATE `account` SET `balance` = 1 WHERE `id` = 1234;`

4) after this query

SELECT `executor` FROM `account` WHERE `id`=1234

gives result 2727

after several repetitions of points 3 and 4 we get result executor=NULL... or 0, as I write the result to a variable with type int, but this is likely all the same NULL... but it is not so important.. in any case executor ceased to be a value 2727

further transaction is rolled back... if again repeat operation, everything is fine without resetting value of executor...