Why did the lamp beads turn black after using glue

We often encounter lamp beads turning black, which leads to the overall brightness of the lamp and causes significant losses to customers. There are only two reasons for the lamp beads turning black. The voltage is too high, and the glue inside the lamp beads is vulcanized. Therefore, we need to eliminate the problem and observe without applying glue. However, this situation is closely related to the quality of the lamp beads.