Regression no, the issue was reproduced in Windows //UE5/Release-5.0 CL 20280985
repro rate 3/3
After the user tries reimport mesh with nanite enabled from mesh editor the mesh was reimported with nanite disabled.
Actual result: the mesh was reimported with Build Nanite disabled
Expected result: Enabled is checked in Nanite Settings section of Static Mesh Details
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-157493 in the post.