This is the same issue as MC-262179.

The bug

If a single unit of tripwire is placed down and is not connected to any adjacent blocks, it will appear as an orthogonal line. This is inconsistent with other blocks with similar such connection behaviour: they either appear as a single centered unit (iron bars, glass panes, fences, walls, redstone wire after toggling) or connected in all directions (redstone wire by default).

How to reproduce

1. Create a floor out of dark blocks
2. Place down a single piece of string

Expected results

The placed string would either appear as a single centered dot (for consistency with most orthogonally connecting blocks) or a cross (for visibility and for consistency with redstone dust).

Actual results

It appears as an orthogonal line instead.

How to fix

This is completely down to the blockstates file for tripwire. This issue can be fixed entirely by editing that alone. I can provide an example resource pack if desired.