If I start a video where I uploaded a 16:9 video for processing the first screen is 4:3 then when I hit play it changes to wider screen. This happens with multiple videos too (e.g. an ntsc and a vga, both are sized to the vga then the aspect ratio on the ntsc changes when I hit play).
Per http://opencast.3480289.n2.nabble.com/JIRA-Ticket-Cleanup-proposal-td7475080.html, this has been bulk resolved as won't fix. If this is still important to you please reopen and we can triage as appropriate.