Hey Tom,
Welcome to the wonderfull world of cutthecrap where every shop’s a rivers raft afloat a stream of luckyshots meandering fields of whoopsydaisies towards the falls of brokeabit.
I cannot help you solve this, I don’t use 360. I however do use this forum and know that it hides a many good posts and solved problems that might relate to your issue,
You seem lucky you have a floater. I’ve seen lots complains about spoilboard dives.
Some topics on this here forum have simular sounding problems like yours and there seem to be knowledgeable peeps on this forum that can help you out.
Maybe they know what is going on and point towards the culprit. Untill then you may like to browse a few topics in the forum like this one..
And maybe stumble onto something that helps you solve your issue? Or even learn something unexpected. Here’s a simple search result on fusion360. You can adjust the search term as much as you like, to home into your issue a bit more, or you can flip through the ones listed.
https://forum.sienci.com/search?q=Fusion360%20order%3Alatest