[MartinH]

I just tried making lines with sharp corners using my PC with a mouse rather than my tablet with a stylus, and I see the same behaviour. So it doesn't seem to be related to either the stylus or the relatively low performance of the tablet. Note that this is happening when zoomed in very close to the page, not when zoomed out:
https://1drv.ms/u/s!An-nEo9boZ4eg-pJ2gne...g?e=cake7p

I'll email you a zip file with the database and xml files.