These two are inseparable now…

Well, why you don’t see Flash with Twilight rn is because he passed away from the Bee (a type of changeling that I thought of… I mean it’s not completely original because I know someone else out there probably thought of bee changelings too) invasion. He died from their poison precisely. So, Twilight wasn’t there to help him (because of princess duties and trying to rule Equestria and stuff),she thought that if she was there for him she could’ve prevented his death… And now she thinks it’s her fault.
So for a period of time, Twilight was depressed (and still is) from Flash’s death. On top of that her son, Dawn Light, is pretty mad at her because she had a choice to stay with her family or continue her work.
Now, (Tempest already being a good friend towards Twilight) she felt sorry for Twi and became closer to her, giving her company. Then after a year or less, they got married, and Twilight was happy again (sort of 😥).

(Okay, Estella was there for her mom then, but she was still too young to understand many things with Twi….. And honestly I don’t know why I gave Twilight a big chest 😅)