  We check the stability of the ABMP16 fit with respect to modifications of
quark PDFs suggested in the recent literature: the strange sea enhancement and
a positive non-vanishing $d/u$ ratio at $x \to 1$. These possibilities are
examined using test versions of the ABMP16 PDF fit which demonstrate no need of
those changes. Furthermore, we localize peculiar features in other analyses
which are responsible for a different behaviour of the PDFs obtained. The
strange sea enhancement can be explained by a choice of the PDF shapes being
not flexible enough and therefore leading to an over-suppressed $d$-quark
distribution. This suppression has to be compensated by a corresponding rise by
the $s$-quark distribution. As a result, an unusually large strange sea
suppression factor is obtained. The non-vanishing value of $d/u\vert _{x=1}$
becomes consistent with zero in case the higher-order QCD corrections to the
D{\O} $W$-asymmetry data, which drive its value, are taken into account.
Furthermore, the related $e$-asymmetry sample prefers a slightly negative value
of $d/u\vert _{x=1}$, although it is consistent with zero. These clarifications
support confidence in the PDF shapes used in the ABMP16 analyses.
