I was unable to find any paper that would validate the rapid-onset gender dysphoria hypothesis.
The paper you link was one of the first to mention a phenomenon called ROGD. You can see that it contains a correction which "serves to provide additional clarifications and context" (my highlight):
Rapid-onset gender dysphoria (ROGD) is not a formal mental health diagnosis at this time. This report did not collect data from the adolescents and young adults (AYAs) or clinicians and therefore does not validate the phenomenon.
As has been pointed out in the comment, the report approached the issue by questioning parents - who may or may not be on board with their child transitioning - if they thought that their child transitioning was sudden. Obviously, the parents assessment is not necessarily correct:
Because this is a study of parent reports, there is some information about the AYAs that the parents would not have access to and the answers might reflect parent perspectives.
Specifically, parents were primarily recruited from websites that were unsupportive of trans people:
Concerns were raised that this study only posted links to the recruitment information on selected sites that are viewed as being unsupportive of transition. [...] Specifically, three of the sites that posted recruitment information expressed cautious or negative views about medical and surgical interventions for gender dysphoric adolescents and young adults and cautious or negative views about categorizing gender dysphoric youth as transgender. And, one of the sites that posted recruitment information is perceived to be pro-gender-affirming.
Children of unsupportive parents may of course not come out to their parents right away, so any transition might seem rapid to the parents.
A more in-depth criticism of Littmans methodology can be seen in Methodological Critique of Littman’s (2018) Parental-Respondents Accounts of “Rapid-Onset Gender Dysphoria”.
There's also this more recent paper, which suffers from the same problems (questioning the parents, who in this case where not supportive, but instead worried their child suffered from ROGD). The paper has also been retracted.
A study from 2021 Do Clinical Data from Transgender Adolescents Support the Phenomenon of “Rapid Onset Gender Dysphoria”? found no evidence for ROGD:
We did not find support within a clinical population for a new etiologic phenomenon of rapid onset gender dysphoria during adolescence.
A study from 2023 Age of Realization and Disclosure of Gender Identity Among Transgender Adults also investigated the issue:
Of 27,497 participants, 40.8% reported “later realization” of TGD identities. Within the “childhood realization” group, the median age of sharing one's gender identity with another person was 20. In this group, the median time between realization of one's gender identity and sharing this with another person was 14 years.
Based on this, they conclude:
The results of this study do not support the ROGD hypothesis.
The World Professional Association for Transgender Health (WPATH) has stated:
The term “Rapid Onset Gender Dysphoria (ROGD)” is not a medical entity recognized by any major professional association, nor is it listed as a subtype or classification in the Diagnostic and Statistical Manual of Mental Disorders (DSM) or International Classification of Diseases (ICD). Therefore, it constitutes nothing more than an acronym created to describe a proposed clinical phenomenon that may or may not warrant further peer-reviewed scientific investigation.
They encourage further research. As seen above, so far that research has results in studies which do not find any evidence for ROGD. I am unaware of any non-retracted studies that would provide such evidence.