Completing the obsidian frame of a Nether portal doesn't activate it if fire has already been placed inside the frame

Relates to MC-233107 and MC-183102. If you start building an obsidian frame for a Nether portal and place fire inside it before completing the frame, the portal won't activate once the frame is finished. You'll need to place fire inside the frame again to activate the portal.

Steps to reproduce

1. Build a nether portal frame but leave out one obsidian block so that the portal wouldn't activate when placing fire
2. Place fire in the not completed frame
3. Complete the nether portal frame

Observed result: The nether portal was not activated.
Expected result: The nether porta should be activated.