How can I stop the borders from disappearing around images when zooming out? (Adobe Acrobat Pro 2017)

In Acrobat, go to Preferences> Page Display> Enhance thin lines (on).

If the frames are stroked lines (vector lines) and not an image or shape, this preference may help.

On the screen? Screen resolution is limited. Somewhere there needs to be a compromise between speed and accuracy. Print, however, should always be accurate.