Don't care what you do with it, just don't sell and Skyrim belongs to Bethesda, so keep that in mind.
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"Fixing USSEP since 2022!"
Jokes aside I have no idea why it happens (CK screenshots above) because seat position isn't edited by USSEP (now it may be, but eh fix is a fix). ESLified, no ITMs, compatible with any USSEP version and edits just one specific seat on one specific carriage. Solitude overhauls are most likely compatible.