I don't know that there is a default path for downloads. That is all controlled in the right-hand panel on the import dialog. I have found that on occasion an update will cause Lightroom not to remember where I was previously downloading. But that doesn't prevent me from resetting the destination to where I want, and that includes other hard drives and even external hard drives.
The LR default on Mac OS appears to be to save to the "Pictures" folder on the system hard drive, which I do not want. So when I set up a new catalog, I set up the download path. Then when I open the catalog and import new images, they go into that path, with subfolders by date. That's the way it has worked until now. I keep all my content on external drives. My complaint is that for no good reason the last update reverted to the "Pictures" default. As I noted, I did a large-ish import expecting it to go to the path on my external drive, as always. But the new update reverted to the "Pictures" folder default, and I have spent a lot of my evening reconciling this. Pardon me if I do not check the various preferences I have set for my preferred workflow to find out which ones the latest update randomly reset.
Usually the download path is reset to your Pictures folder if you open the import dialog at a moment that the destination disk is not available. An update does not do this, so it’s probably purely by coincidence that it happened after an update.
By the way: do you know you can save this path and other settings as an import preset at the bottom middle of the import dialog? If you do this and use these presets, you never have to worry that Lightroom changed something without you noticing it.