I sometimes get longitudinal lines on the base, even with fairly light pressure against the base. I think the marking is possibly from the metal filings falling down, and/or the black gunk that sometimes builds up on the angle guide. I can't really predict when marks will appear. It may be more noticeable on freshly ground bases with good structure than on older, more worn, bases.
If I wipe the angle guide clean with a cloth every few strokes, and scrape the gunk whenever it appears, I can really minimize the issue, but it's pretty time consuming. I suppose always cleaning the base first might help eliminate the black gunk, but I don't always have time to do an edge tune and a wax on the same evening, which might be my error.
This is worse with the orange Multi Tuner (Mountain TEK, but I've seen it rebranded elsewhere) which I only use when working the base edge, as it concentrates the downward sharpening pressure in a half-dozen narrow stripes. Thankfully I rarely need to get into that kind of work.