Studies on gender dysphoric young people often suffer from high rates of loss to follow-up – which could skew transition satisfaction rates.
In one study  of 77 pre-teen participants, 30% were lost to follow up by their teenage years: either they did not respond to the recruiting letter, or were not traceable. In another study , as many as 75% of participants were lost to follow up.
An excellent précis of this problem can be found in a 2018 paper , which gives further detail:
Smith et al. report that sex reassignment is effective, based on a study of 162 adults who had undergone SRS. They were able to obtain follow-up data from only 126 (78%) of subjects because a significant number were “untraceable” or had moved abroad.
De Cuypere et al. report that sex reassignment surgery is an effective treatment for transsexuals. Of 107 patients who had undergone SRS between 1986 and 2001, 30 (28%) could not be contacted and 15 (14%) refused to participate.
Johannson et al. reported good outcomes for SRS. Of 60 patients who had undergone SRS, 42 (70%) agreed to participate in the follow up research. Of the non-participants, 1 had died of complications of SRS, 8 could not be contacted and 9 refused to participate.
Salvador et al. reported that SRS has a positive effect on psychosocial functioning. Only 55 of the 69 patients (80%) could be contacted as 17 were lost to follow-up
Van de Grift et al. reported 94–96% of patients are satisfied with SRS and have good quality of life. A total of 546 patients with Gender Dysphoria who had applied for SRS at clinics in Amsterdam, Hamburg and Ghent were contacted to complete an online survey. Only 201 (37%) responded and completed the survey.
A good example of how this phenomenon can affect satisfaction and regret statistics comes from a 2018 paper , which is often cited as proof of low regret rates. The loss to follow up rate in this paper is 36%. The authors also state:
In addition, in our population the average time to regret was 130 months, so it might be too early to examine regret rates in people who started with HT [hormonal treatment] in the past 10 years.
 Wallien, M.S. & Cohen-Kettenis P.T. (2008) Psychosexual outcome of gender-dysphoric children. J Am Acad Child Adolesc Psychiatry 47 (12): 1413-23. [Link]
 Rauchfleisch, U., Barth, D. & Battegay, R. (1998). Resultate einer Langzeitkatamnese von Transsexuellen. Der Nervenzart 69: 799-805. [Link]
 D’Angelo, R. (2018). Psychiatry’s ethical involvement in gender-affirming care. Australasian Psychiatry 26 (5): 460-463. [Link]
 Wiepjes, C.M., Nota, N.M., de Blok, C.J.M., Klaver, M., de Vries, A.L.C., Wensing-Kruger, S.A., de Jongh, R.T., Bouman, M.B., Steensma, T.D., Cohen-Kettenis, P., Gooren, L.J.G., Kreukels, B.P.C. & den Heijer, M. (2018). The Amsterdam Cohort of Gender Dysphoria Study (1972-2015): Trends in Prevalence, Treatment, and Regrets. Journal of Sexual Medicine 15 (4). [Link]