I think, it is not a bug. A tab character (hex 09) is by default interpreted with 8 spaces by ALL programs. UltraEdit lets the user specify by how many spaces a tab should be interpreted based on the file extension. The text in the output window is just something in RAM memory, not a file. So the default is used. UltraEdit cannot know how a human editor has seen the tab during the file creation.
If you get the find output from your source files, you want that UE displays it with two spaces. And what will happen, if you use Find In Files on different files, not created by yourself?
Excuse me, but I personally think, that it is stupid to use a tab character with a width setting of two spaces. Why not simple use 2 spaces instead of a tab? If a tab is interpreted with more than 3 spaces - OK, but 2 spaces? That makes no sense. That's my personal point of view.
However, you can write a feature request by e-mail to IDM for a special tab setting for the output window. But don't write, it is a bug.
Or you run a Replace In Files and replace ^t by 2 spaces and problem is solved.