Part of object did not print

I’ve been 3D printing for several years but received my P1S last week and have made a dozen prints with no problems and am amazed at what this printer can do. However, I tried to print a poop-chute from a model I got off MakerWorld and experienced an unusual failure I have never experienced with my older printers. The main post that would hold up the chute when in use failed to print entirely. It appears in the print preview in Studio but the printer didn’t even try to print it until it got close to the main body of the chute. I didn’t notice during the print because I used tree support and I thought one of the various tree “trunks” was that main support.

The model did give me trouble placing on the build plate so that I would not get over-edge warnings and I went with tree support because I got several edge warnings when I used “normal” supports in Studio. I also got a warning about too many triangles (over 1M) and the suggestion to “simplify model”, so I let Studio simplify the model and, along with tree support, eliminated all errors. As you can see in the photo the main chute printed beautifully, but the post is missing.

Any ideas what would have caused this?