I don't know who else suffered this bug, but driving through the sand dunes in Rally Adventure made my normally steady 72 fps tank to low-40s with horrendous choppiness. The only cure for this problem was lowering the Deformable Terrain Quality to as low as needed (some said to turn it off, others said to lower it down one step) to get my framerate back. That soured my enjoyment of an otherwise really good DLC, because that closes the doors to 25% of an already small map. (Before anyone asks: my specs are good enough. I have never struggled with performance in FH5 before, steady 72fps with all settings on Ultra or Extreme. This definitely isn't on me.)
So I decided to rush through every event I needed to get the Goliath and ... not have to bother with Rally Adventure ever again. Back to Mexico for me. Real shame, honestly. They should bring the Rally mode to the main game, the pace notes and the excitement of not knowing how you did until the very end, only to get a very narrow win, and then another, and then another, with maybe a very narrow loss here and there... That was a breath of fresh air. Sadly, performance problems of this amplitude ruin the mood.