I wonder if it's coincidence or not, but I once grew a mango from seed in the US, kept indoors, and it did similarly - grew great for about a year (got to about 2 foot tall), then got sick and died. I wonder if there's something in the US that mango trees arn't used to dealing with?

Very interesting to watch grow though - each set of leaves starting out floppy and purple then stiffening up and turning green, just like the one in the video.

I just planted the whole mango pit as-is, without removing the inner seed, and it had no problem germinating.