At first I thought it was due to using the keywork DateTime as the field name, but I tried it with a different field name and it did the same thing.
Change it to
AND TTOC(dt.datetime)=TTOC(crMaxDt.datetime)
Works every time. No idea why the datetime join doesn't though.
Sounds like a bug.

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>It appears to us that SQL Selects involving DateTimes in Join conditions produce inconsistent answers depending upon the data. We are attempting to extract the latest transaction DateTime for a given transaction type in a table. The following simplified data table and code will reproduce the problem.
>
>Please run the following code repeatedly and observe the results in crWrongAnswer. It should always have three rows, but you may observe anything from zero to three rows.
>
>** Begin Code
>CREATE TABLE DT (DTID I, FKID I, DateTime T)
>SELECT DT
>INSERT INTO DT (DTID, FKID, DateTime) VALUES (1, 4, DATETIME()-INT(RAND()*5555555))
>INSERT INTO DT (DTID, FKID, DateTime) VALUES (2, 5, DATETIME()-INT(RAND()*5555555))
>INSERT INTO DT (DTID, FKID, DateTime) VALUES (3, 6, DATETIME()-INT(RAND()*5555555))
>BROWSE LAST NOWAIT
>SELECT FKID, MAX(DateTime) AS DateTime FROM DT GROUP BY 1 INTO CURSOR crMaxDT NOFILTER
>BROWSE LAST NOWAIT
>SELECT DT.* FROM DT INNER JOIN crMaxDT ON (DT.FKID = crMaxDT.FKID and DT.DateTime = crMaxDT.DateTime) INTO CURSOR crWrongAnswer NOFILTER
>BROWSE LAST NOWAIT
>** End Code
>
>This is driving us crazy in Virginia. Please help!
>
>Thank you,
>John L. Malinowski
>Wicks Broadcast Solutions