Each /target line is being evaluated. So, in fact, you are targeting each character in order if they are alive. This means the last name on the list who is alive will be who is targeted.

What you may want is the following:

/tar [target=wyldvyne, nodead] [target=rowdyroot, nodead] [target=leafcutter, nodead] [target=savagesprout, nodead] [target=thundertwig, nodead] [target=wyldvyne]

This way once a condition inside the [] is true that is who you target and the remaining conditions get ignored.

Note: I added the last one so if they are all dead then they will follow wyldvyne.

Your way would technically work if you reversed the order although that following ; may be an issue, not sure.