From:  Michael Gibson
4774.2 In reply to 4774.1 
Hi eric, thanks for reporting that - yeah that looks like a bug, it is putting the joined result on the active style instead of maintaining the previous states.

For surfaces it works like you expected - for example if you have 2 blue surfaces if you join them together the joined result stays blue like you wanted. So it would be good to have curves work in the same way, I will tune that up for the next v3 beta.

- Michael