Kerbal Space Program

The RFP-F flag and any sized curved flags in the landscape orientation have their colliders enabled in flight, but only if you choose the maximum flag size.¶

Here's how to reproduce this:¶

1. Take any curved flag in the portrait orientation and make a simple craft from the screenshot №1 (or you can take mine, I will attach it below), so the flags will go through the ground when you spawn it.

You can choose literally any size of the curved flag (from RFP-0 to RFP-4), as long as it stays in the portrait orientation. Launch the craft and observe its behavior. Any flag size and variant you choose, it goes through the ground, as expected. And it also will not collide with any other parts or kerbals in flight. As expected.

2. Now, take the same craft (or take the second attached one) and change the orientation of the flag to landscape and set it to the maximum size if you didn't already, as shown on the screenshot №2. Launch it and observe. It will spawn standing on the flags, because now they have colliders in flight.
This happens with the curved flags only in the landscape orientation and only when the maximum size variant is enabled.

3. Let's move to the flat flags. You can take the same craft and replace the flags on it to the RFP-F ones, as shown on the screenshot №3. The same steps apply. Make the flags large, launch the craft and look.
Flags go through the ground and will not collide with anything.

4. Change the flag size to Huge (the maximum size variant), launch the craft and watch. It will stand on the flags as well.