I tried looking for similar issues but didn't really find anything close. I have webcore pistons that control my hue bulbs at certain times of the days. 99% of the time, they work but there's that one percent I can't nail down.
An example was this morning. The bulb is supposed to turn off at a specific time, which it did, then a few seconds later it turned back on. I immediately looked at the device log but there's no entry for when it turned back on. A similar even happened the night before, I have a set of Hue Iris that turns off after no motion, the automation happened but the same thing happened, it turned back on after a few seconds with no entry in the log.
There is no Hue Bridge log that I am aware of. However, the Hubitat device (from Hue which you are using in webCoRE) does keep event history in Hubitat and may have clues. As far as I can tell, Hubitat's stock Hue Bridge integration doesn't update device state until it hears something back from the Bridge, though I don't know exactly what it looks for in that response (probably Hue's "success" message but maybe anything besides an HTTP error code or JSON error)--as opposed an "optimistic" approach one might take by just assuming the command works and updating the device state immediately--so this may or may not actually help you. If it does, it would at least offer some insight into what the hub thinks is happening. However, that could be either because you sent a command to the device from Hubitat and it (probably) succeeded, or something else changed the bulb (Hue app, Alexa, or another integration) and Hubitat caught the updated state on a poll/refresh.
To get a better idea of when your automation runs, you could try a few ideas: logging in webCoRE for the piston, adding a virtual device to be manipulated along with your Hue devices (to see if the same thing happens to it), or another creative idea someone might have. Hue has the added complication that something outside Hubitat could also be manipulating it, so I'd check those third-party integrations if you haven't already too (IFTTT applets, Alexa routines, Hue app schedules, etc.).
