It's not a new behaviour. It also happens with 4.2.7.
When selecting areas with contiguous selection tool, these unexpected lines of marching ants appear. I didn't write a bug report because it doesn't produce any actual problem on doing the job, but it seems weird.

Just wanted to share.

Nightly version : git eb5dfa0 (4.3 prealpha)

I haven't managed to reproduce that bit of weirdness, but there's defnitely something off with the contiguous selection tool. If you have a .kra file with which you can always reproduce this, please do report a bug and attach the file.

It's not specific to some files. You want me to just attach any file from my system?

Also, these 'lines' are flickery and you can only 'see' those on particular zoom levels, depending on the case.

I tried to reproduce on my own system and couldn't, please attach the file and note the zoom level

Turns out it does not only happen with contiguous selection tool, and it seems the 'lines' are connected in the order you made the selection areas.

I haven't manage to reproduce this with outline selection tool yet.

Bug report : https://bugs.kde.org/show_bug.cgi?id=413220