Clipping masks make areas of any objects behind disappear
This looks like a new visual bug. There seems to be a problem with the way Illustrator is displaying clipping masks. It has only started happening recently.
The image below is taken from a file that worked fine before.
The wavy lines are contained inside a clipping mask (the same shape as the blue section). I have included images of the lines outside of the clipping mask (above) and inside the clipping mask (below).
Any separate objects placed behind this clipping mask have a section missing (see black shape behind). The clipping mask and the black shape are completely separate. The black shape is just a rectangle.

This only happens with clipping masks that had been created in the past. Any newly created clipping masks do not have the same problem. However this is quite a big problem for us as we have hundreds of files, each with multiple clipping masks. It won't be possible to recreate them all.
This is only a display issue. Any exported images don't have the missing sections.
I have the same problem on my 2019 iMac in the office and my 2024 iMac at home. One of my colleagues has the same problem on a 2017 iMac. Another colleague does not have the same issue on a 2020 iMac.
