Welcome to Seamly!
This difficulty was actually mentioned a couple days ago in this post:: Draw paralelle in draft modus - #3 by Douglas
In short, if you want your seam allowance to flare, you have to draft the flare in.
Although it’s often handy to abuse the seam-allowance feature to avoid drafting a hem, a hem is not actually a seam-allowance, & should be drafted in, much like the sleeve itself. The Mirror Objects by Line tool is often handy in this spot.
I hope that, (particularly the linked post,) is helpful. Please ask for clarification as needed. Happy drafting!
