This problem started in 4.24 and not in 4.23.
CL 9114926 seems to be relevant.

Steps to Reproduce

Import the FBX file into the project, and set the Normal Import Method of Import Setting to Import Normals and Tangetns.
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It is correct when imported, but not when re-imported.

sample : [Link Removed]
1. import
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2. re-import
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The number of vertices changes with each re-import
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Cannot Reproduce
CreatedJan 31, 2020
ResolvedMar 13, 2020
UpdatedJun 23, 2020