Cannot override Timeout directives in included configuration

Redefining TimeoutIdle or TimeoutNoTransfer in a configuration file inside /etc/proftpd/conf.d/, after they've already been defined in /etc/proftpd/proftpd.conf, fails to change those values for the daemon — that is, after restarting the daemon, the values for those parameters remain as those set in /etc/proftpd/proftpd.conf.

At least TimeoutIdle or TimeoutNoTransfer are affected. TimeoutSession and TimeoutLogin, which (in stock 16.04) haven't been defined in the master configuration file can be set effectively, but setting them first in the master file, then again to different values inside conf.d reveals them to be equally affected.

On the other hand, (at least) ServerName *can* be overridden in the custom configuration file, so the handling of different configuration knobs in this regard seems to be inconsistent.

Steps to reproduce:
1. Create a /etc/proftpd/conf.d/proftpd.conf with the following lines:
TimeoutIdle 10
TimeoutNoTransfer 10
2. Restart proftpd
3. Log into the server, wait 11 seconds

What I expect to happen:
To get kicked out of the server.

What happens instead:
I'm allowed to linger on the server, presumably for the 600 seconds defined for TimeoutNoTransfer in /etc/proftpd/proftpd.conf.

ProblemType: Bug
DistroRelease: Ubuntu 16.04
Package: proftpd-basic (not installed)
ProcVersionSignature: Ubuntu 4.4.0-33.52-generic 4.4.15
Uname: Linux 4.4.0-33-generic x86_64
ApportVersion: 2.20.1-0ubuntu2.1
Architecture: amd64
CurrentDesktop: Unity
Date: Fri Jul 29 20:29:11 2016
InstallationDate: Installed on 2010-01-15 (2387 days ago)
InstallationMedia: Ubuntu 9.10 "Karmic Koala" - Release amd64 (20091027)
SourcePackage: proftpd-dfsg
UpgradeStatus: Upgraded to xenial on 2016-04-19 (101 days ago)