DORMANT is a keyword used in Linux. For an explanation of the output in your question, consider the following output that state is UP and mode is DORMANT

Mode Type

The default mode type is called DEFAULT. When an interface is in DEFAULT mode, the interface state transitions to UP when the following condition is met:

Link is detected

There is also a mode type called DORMANT. When an interface is in DORMANT mode, there are additional criteria for the interface state to transition to UP. In DORMANT mode, the interface state transitions to UP when both of the following conditions are met:

Link is detected
MLAG has finished setting up everything for the bond

Link State

In the context of link state, DORMANT indicates the interface is not in a condition to pass packets but is instead in a pending state waiting for some external event. For an interface to be usable for forwarding, it should be in an UP state as it is in the output shown above. Content retrived from cumulusnutworks

Looks like it is used for power saving thus why it still works fine. From https://tools.ietf.org/html/rfc3132

  Dormant Mode - A state in which the mobile restricts its ability
  to receive normal IP traffic by reducing monitoring of radio
  channels.  This allows the mobile to save power and reduces
  signaling load on the network.

Even looking at https://ubuntuforums.org/showthread.php?t=2362438 also shows when power saving modes are enabled the mode switches to DORMANT.

Hope this helps!