I dont really agree on transition being short period but maybe thats because I see transition in diffrent levels of states.
from just being accepted male/female, or living in a role who more masculine/femenine to look up information, comming out, saving up money, having all kinds of questions to be answered, and for some, homones,surgery changing paper stuff and so on.
why people disapear? IDK,
as already mention alot probably go stealth when they have the chance. and alot of people move on with other things who arnt transrelevated. I know from myself now that I focus alot more on "normal problems" such as education and work, than I did before because transition have ment so much in my life, and now when I pass I feel I get more time for those things.
I guess also some people could need a break from the whole transscene once in a while, and if they had a long break maybe they feel less needed for comming back.