A particular set of piano rolls were recorded before 1913, but to my knowledge were not published until 1989. The recordings are piano transcriptions of symphonic works, performed by their composer/arranger. The symphonic works in question were published well before 1910 and are currently in public domain. I am interested in transcribing the rolls to study the arrangement techniques of the composer. I would be using this information to reconstruct his arrangements of other works, and am interested in publishing my findings as it's for a thesis project. If I were to refrain from publishing the direct transcriptions, and considering that the symphonic works are in public domain, would I be violating copyright by transcribing the rolls for study? Is it possible that the rolls could be considered public domain as they are arrangements of public domain works published postmortem?
First: the real answer to your questions is "consult your academic library staff." If this is for a thesis project, then you're presumably affiliated with a college or university, and the librarians at such institutions are used to helping sort out copyright claims and helping researchers obtain the necessary clearances.
There are two questions you're asking, really.
Are these works in the public domain?
Possibly, but not for the reasons you think. In the US, copyright in unpublished works lasts for the life of the creator + 70 years. After that, they pass into the public domain. This means that if the composer/arranger died before 1919, then the works passed into the public domain before the formal publication in 1989, and they are still in the public domain now. However, if the creator died after 1919, then their 1989 publication effectively "resets the clock" on the copyright, and they will not come into the public domain until (according to the above link) 70 years after the death of the creator or 2048, whichever comes later.
What doesn't matter is that the original symphonic works were in the public domain. Even if the 1812 Overture is in the public domain, I could write an arrangement of it for accordion quintet and I would have the exclusive right to publish it and profit from that arrangement for the standard term. In your case, if the work really is a straight transcription, it might not have a copyright; but if the work required some amount of creativity and artistic decision-making (which a piano transcription of a symphony certainly would) then the arrangement gets its own copyright when it's published.
Can I make transcriptions of them if I don't publish them?
Probably, but you should really consult your librarians. It's entirely possible that your proposed copying would fall under "fair use"; it would be for scholarship & research and you're not planning to include the transcriptions in your final published work. On the other hand, you're talking about effectively copying a substantial portion of a creative work that is still in print, which argues against fair use. Fair use rules are notoriously flexible/vague, and so it would be worthwhile to consult with an expert on the subject (i.e., your school's library staff.)