I recently opened a pull request on the official srfi 134 reference implementation, as there are two errors. These errors are also in Sagittarius.
In your file: sagittarius-scheme / sitelib / srfi / %3a134 / ideque.scm
line 301 should be: (check (- lenf n) (drop f n) lenr r) to keep the size of f correct
line 306 should be: (unless (<= 0 n (ideque-length dq)) so take/drop can use all elements in the deque
There are additional test cases in the attached file, to highlight and check the fix.
(Sorry for not making a pull request, but I seem to be having problems with Mercurial.)