With this configuration, VLAN tagging is preserved.

set interfaces xe-0/0/19 description "SPAN-SOURCE"
set interfaces xe-0/0/19 unit 0 family ethernet-switching interface-mode trunk
set interfaces xe-0/0/19 unit 0 family ethernet-switching vlan members PROD
set interfaces xe-0/0/19 unit 0 family ethernet-switching vlan members DEV

set interfaces xe-0/1/2:3 description "SPAN-OUTPUT"
set interfaces xe-0/1/2:3 unit 0 family ethernet-switching

set forwarding-options analyzer SPAN input ingress interface xe-0/0/19.0
set forwarding-options analyzer SPAN output interface xe-0/1/2:3.0