Ideally should work as before, where the particle remains fully opaque until the duration gets the transparency below 1

What's the fromTransparency and toTransparency for this behavior?

toTransparency is a value basically a smidge above 0, something like 0.00001
However, fromTransparency is a value between 2 and 8 depending on the application, because this keeps the particle from fading away until the very last moments of its duration. I believe it's 8 here for the tire marks though, IIRC.

To clarify, if we have a particle with a duration of 60, and a fromTransparency of 3, the particle will should remain solid for two seconds and will fade away within one second. Specifically the bug is that it no longer does this, at least in 1.16.5
This is very useful for specific particles like bullet casings or tiremarks.

Uuuh, new bug. 😄
Now sizes are clamped, which wouldn't be so bad except for the fact that they're clamped to 1. This doesn't affect handguns or exhaust smoke any, but try backfire smoke or explosions, or large fires and it'll become very obvious.

(scale issue)
Fixed in latest 22.6.0-beta (As of 6:47pm)

edit: And transparency is fixed too 😛

Fixed in V22.7.0.