sketch planes in assembly and part modes don't match

In an assembly, if you created the parts in a multi-body part environment then often the only constraints you’ll need are flush constrains to each of the three main sketch planes. X flush constrained to X. Y flush constrained to Y. Etc.

However if you create the parts individually in their own individual part files then flush constraints to the main planes usually won’t work. The main reason being the parts are usually created directly on their main XYZ axis and not necessarily in the same orientation. In this case, in the assembly, they will require other diverse constraints relative to other parts and not the main planes. Furthermore, the relative positioning and orientation of the part planes and the assembly planes is usually irrelevant in this case.

In your case, the part was inserted and then rotated relative to the main planes. How it got in this orientation is somewhat irrelevant. How you reorient that part relative to the main planes is entirely up to you and should be done cognizant to the original method and environment of the part file as described above. Never depend upon a part file just automatically being in the correct orientation and location. It probably won’t be and subsequently can be easily moved and rotated. Parts should always be constrained in the proper location and orientation.