I believe there is a bug when using “Happens daily at” with a specific “Date & Time”. For some reason the piston executes twice (within the same piston event) causing undesirable results.
Thanks to a suggestion by @WCMore I found that putting a “2 second wait” as the first statement in the piston it avoids the issue. Of course this is timing dependent so not sure if it will fix all occurrences. It can also be “fixed” by qualifying the “happens daily at” with a time window of about 1500ms.
Also, just as FYI when reading through the threads on “Date & Time” I found a suggestion that it triggers daily … this is not true, it only triggers on that specific Date & Time (which is good news!)