Did you delete a character before TTK? Characters deleted before the moments of triumph were complete, could cause triumphs to be lost.
It was a while ago, and order of events can be forgotten - for example, the patch that removed the elemental burns from skolas, was released in June 2015:
https://www.bungie.net/en/News/Article/13033/7_Destiny-Hot-Fix--06152015
Moments of triumph year 1 weren't announced until July 7, 2015 - so it wouldn't have been possible to confirm the MoT completion after defeating Skolas on solar burn.
Unfortunately, if the node is incomplete, there is no means of completing it anymore - those triumphs were forever locked down in September 2015.
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Eh, I can't remember all details. I swear I had it done. Maybe I delet my warlock early and that's why. I should be compensated with recon armor Bungie pls