will a lab feel deterred from hiring me if my undergrad thesis is in a different subfield?
Since you have some history there, it seems very unlikely that such a thing would be a detriment. In fact, it might be viewed as positive that you aren't &quot;too narrow&quot; in your education and outlook.
But, it is up to them and you only learn by applying.
And picking a topic for such a narrow reason as obtaining a short term internship would be, I think, a mistake.