Selecting two objects adds millions of points to second path...
Adobe Illustrator 24.2.1 (and its predecessor) have a horrible feature which I assume is a bug. When selecting two separate paths (or even two separate text objects) using the Selection Tool or Direct Selection Tool, it seemingly adds millions of points to the second path. The points are not really added and don't actually exist, but they LOOK like they do, meaning it adds a thick blue line along it that looks ghastly and is completely off-putting. I can't use those versions at all and have had to stick to 24.1.3 which is the latest version to not do it. I'm running it on a 27-in 2019 iMac with the lastest version of Catalina OS. I have heaps of RAM and oddles of SSD space. As an aside, the text cursor is STILL too thin to see well, despite numerous bug reports and complaints! Baffled as to why you refuse to fix it or even address the complaints in a sensible fashion.
The description matches with the fixed 'auto-keying' issue.
Ninian, the thick blue line is the way Illustrator marks the 'key object', which is used for the aligning. The fact it’s showing up when you have more than one object selected is officially a bug, introduced in 24.2, but almost fixed in 24.2.1 (normally you would have to single click an object in your selection you want to make a key one).
Are you sure you are updated to 24.2.1?
If yes, please try to open Align panel and manually change the 'Align to' mode to 'Artboard', and then try to work as usual. If the bug with auto-keying is still there after it, please record it on video (including Align panel look), and send over email@example.com, to help the team to finally fix it.
As for the cursor issue, please log it as a separate feature request (I can’t find one).