[FFmpeg-user] sequence with alpha over a still renders alpha incorrectly

[Ehren Addis]

I'm trying to put a png sequence with an embeded alpha over a still 
image with burned in timecode.  The following has been had from 
everything I see on the web (with the exception of using -shortest that 
never worked).  This renders, but the alpha channel renders in a strange 
banded sort of way.  A smooth gradation comes out like lots of 
transparent stripes that move around.  I'm starting to suspect this 
isn't the way the current ffmpeg would do it.  Can anyone point me in 
the right direction for the proper syntax?

ffmpeg -loop 1 -r 29.97 -i background.jpg -r 29.97 -i 
foregroundsequence%05d.png -filter_complex "overlay, 
drawtext=fontfile=ARIAL.ttf: timecode='00\:00\:00\:00': r=29.97: 
x=(w-tw)/2: y=h-(1*1h): fontsize=32: fontcolor=white: box=1: 
boxcolor=0x00000099" -r 29.97 -t %dur% -vcodec prores -profile:v 2 -y 
output.mov

I had to use -t instead of -shortest to get this to not encode 
indefinitely, contrary to examples floating on the web.

Thanks!