Beginning with 0.8.9, the transparency setting is being ignored when using the --color-bg option. The background will now end up as a solid plain color without any transparency. This problem started with 0.8.9 and was not present in 0.8.8 or older versions.
Hi Christian, thanks for the report! I can reproduce the problem and will look into it.
Igor referenced this bugreport in commit b3b0ec71367e6b972d361d4beb391e064e89dd03 Fix 'Transparency setting ignored when using --color-bg' https://git.xfce.org/apps/xfce4-terminal/commit?id=b3b0ec71367e6b972d361d4beb391e064e89dd03