This happens here in Thailand too.

Clipped through here at Mushroom Castle.

Clipped through here on the same map.

Flash Jumping/Soul Rush also causes you to fall through the ground a lot in the Kerning Swamp along the bottom flooring. It's only occasional but it does happen. Flash Jumping into the stairs going up to Eos Tower leads to you clipping through them sometimes. Flash Jumping through the Ant Tunnel sometimes clips you through the ground.

I will also comment that the Warrior Rush Skill also clips through the ground in many locations where the ground is uneven. The platforms around Perion, the platforms around Ellinia, the platforms in Kerning Swamp, pretty much everywhere in the Ant Tunnel.

Thanks to everyone for the locations. Two causes: on slopes your character was only pulled partway onto the ground each frame, so at Rush and Flash Jump speed you sank into rising slopes; and a Flash Jump hop off the low end of a steep slope was refused its landing over a rounding difference. Both are fixed. I tested 15 real spots (Perion, Kerning Swamp, Ant Tunnel, Thailand) and swept 50 maps from your reports. Please tell us if you still find a spot.