When importing an FBX with a known FBX import issue and the Message Log already open, the FBX Import error will appear but the FBX Import Category which is not present upon opening the Message Log will NOT appear until you close and reopen the Message Log.
On First Opening Message Log:
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On Importing the FBX:
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On Closing and Reopening the Message Log:
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Also Tested and Confirmed in \Dev-Main\CL-2794368
RESULTS: Degenerate Tangent Message appears but no FBX Import Category
EXPECTED: Either the FBX Import Category is always present in the Message Log Window or it appears even if the Window is already open
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-24266 in the post.