During the latest month I've experienced a problem never seen before: lines appear and disappear when I'm zooming.

In the first picture, two dash-dot cross lines are visible. In the second, with a little zoom, they are not visible anymore.
Zoon out, they appear; zoom in and it disappear.
I found this problem in each file, in particular, for thick lines but not only. Sometimes, only the dots of a dash-dot line disappear... but without a logic reason or a specfic event, for what I can understand.

What can I do to solve this boring matter? What i have to set in a different way?

Thank you!

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