It is easy to replicate. Just load up Sigil 0.5.3 then in Book View do a split. Depending on whether Tidy is turned off, the "previous" page of the split will have this cruft either directly applied to the body tag or as an inline body style. The page created "after" the split point does not have it in.
And I knew about it because I specifically fixed it for 0.5.90x
