Should my master's thesis be related to my future PhD research?
Can you ask your prospective PhD advisor? Surely it is not impossible to change areas between masters and PhD, but for some topics some earlier experience may be very helpful. So it depends.
As a supervisor, I'd not normally expect a PhD student to have worked in the same area before, although I had two PhD students who had done this, and it was actually a good thing; and in one case I wish a student would have learnt more about the area of their topic before the actual start, but you can do that even if your thesis is in a different field. Most students who &quot;switched&quot; had successful projects as well, so I wouldn't expect this to be a big problem, but there is still a possibility that a supervisor prefers a student with more experience in the same area, if available.