There are several reason a bulb can appear to be "online" even after being turned off. Most often, it is just due to the Sengled Home app not being refreshed or the internet connection of the device used to view the Sengled Home app. Second, the Sengled Element bulbs, when they are used on dimmer switches, will not report properly.
Remember, Sengled Element bulbs, like most LED bulbs, cannot be used on dimmer switches such as wall dimmers, ceiling fan fixtures or 3-way lamps.
Sengled Element Plus bulbs, however, can be used on certain wall dimmers; fore more information about this, please visit the following link: https://support.sengled.com/hc/en-us/articles/115003534408-Compatible-Dimmers
Furthermore, Sengled Element bulbs, like almost all LED bulbs, cannot be used in fully-enclosed fixtures. Due to humidity concerns, all Sengled Element bulbs, with the exception of the Sengled Element PAR 38, are not meant to be used outdoors.
A third reason would be due to the internet connection of the hub to the bulbs. Reporting can be delayed as much as ten seconds under normal conditions.
Fourth, if the bulbs are too far away from the hub when they are switched off, they will not have enough power to send a signal that reports their status as "off".
The Sengled Home app makes no distinction between bulbs that are powered off from the Sengled Home app, from the light switch or removed from the light socket altogether.