Thin lines in design when pasting from Ai to Ps

Is the Illustrator doc using a pixel grid? If not it may be best to ensure that the tile is using whole pixels, not fractions.

Otherwise it may be a transparency and or anti-aliasing issue, you can always try exporting the pattern fill as a raster directly from Illustrator, or even rasterizing the artwork directly in Illustrator/copy/paste/then undo the rasterization.

It is an Illustrator Pattern not a Photoshop pattern.

You can zoom in more and you will find that it is just a preview lines.

Don't make {Expand} for your pattern. or you can copy the pattern unit and go to photoshop to make a pattern there.

It looks like it's from the pattern. You can recreate the pattern in PS... How are you viewing the image? Sometimes you can get artificats at different zooms - is this 100%?

[moved from Adobe Creative Cloud to Photoshop]