This article seems relevant (if someone can find a less terrible pdf, I would appreciate it). Abstract:

The illusion of independent agency (IIS) occurs when a fictional character is experienced by the person who created it as having independent thoughts, words, and/or actions. Children often report this sort of independence in their descriptions of imaginary companions. This study investigated the extent to which adult writers experience IIA with the characters they create for their works of fiction. Fifty fiction writers were interviewed about the development of their characters and their memories for childhood imaginary companions. Ninety-two percent of the writers reported at least some experience of IIA. The writers who had published their work had more frequent and detailed reports of IIA, suggesting that the illusion could be related to expertise. As a group, the writers scored higher than population norms in empathy, dissociation, and memories for childhood imaginary companions.

The range of intensities reported by the writers seems to match up with the reports in r/Tulpas, so I think it's safe to say that it is the same phenomena, albeit achieved via slightly different means.

Some interesting parts from the paper regarding dissociative disorder:

The subjects completed the Dissociative Experiences Scale, which yields an overall score, as well as scores on three subscales:

• Absorption and changeability: people's tendency to become highly engrossed in activities (items such as "Some people find that they become so involved in a fantasy or daydream that it feels as though it were really happening to them).
• Amnestic experiences: the degree to which dissociation causes gaps in episodic memory ("Some people have the experience of finding things among their belongings that they do not remember buying").
• Derealisation and depersonalisation: things like "Some people sometimes have the experience of feeling that their body does not belong to them".

The subjects scored an overall mean score of 18.52 (SD 16.07), whereas the general population score a mean of 7.8, and a group of schizophrenics scored 17.7. Scores of 30 are a commonly used cutoff for "normal" scores. Seven subjects exceeded this threshold. The mean scores for the subscales were:

• Absorption and changeability: 26.22 (SD 14.65).
• Amnestic experiences: 6.80 (SD 8.30).
• Derealisation and depersonalisation: 7.84 (SD 7.39).

The latter two subscales are considered particularly diagnostic of dissociative disorders, and the subjects did not differ from the population norms on these. They each had only one subject score over 30 (not the same subject).

What I draw from this: Tulpas are the same phenomenon as writers interacting with their characters. Creating tulpas doesn't cause other symptoms associated with dissociative disorders. There shouldn't be any harmful long-term effects (if there were, we should have noticed them in writers). That said, there are some interactions that some people have with their tulpas that are outside the range (to my knowledge) of what writers do:

• Possession
• Switching
• Merging

The tulpa community generally endorses the first two as being safe, and claims the last to be horribly dangerous and reliably ending in insanity and/or death. I suspect the first one would be safe, but would not recommend trying any of them without more information.

(Note: This is not my field, and I have little experience with interpreting research results. Grains of salt, etc.)