It is a somewhat well-known issue (#4679) that seaborn's styles make "+", "x" markers invisible by default (because they set lines.markeredgewidth to zero). Although it may be helpful to use a separate rcparam to control such markers, this is not trivial (see linked issue). Instead, in this issue, I'd like to argue that matplotlib should not ship styles available by default where some markers are completely invisible; instead, we may e.g. set lines.markeredgewidth to a tiny value, so that the markers are at least faintly visible (and it may be easier to realize where the issue comes from), even though it is not strictly equivalent to the "official" seaborn style.

From simple attempts, it seems that a value like 0.25 should be enough.