I found this Unreal Engine forum thread asking how to randomly flip the UVs of particles at spawn in Niagara. I answered the forum, and I figured id share the solution here as well!
Click the plus to add a module to Particle Spawn.
Type “uvscale” in the search filter and click Particles.UVScale
Click on the new Set Variables module that was created
In the details panel, click the carrot next to Particles.UVScale and search “break” and click on Break Vector 2D.
Click the carrot next to the X input of Particles.UVScale
Search for “uniform aor” and choose Uniform AOr BFloat
Set the A and B of the X value to (-1) and 1 respectively, and set Y to 1
Your get randomly flipped UVs at spawn