Server where the route is not working:

Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
0.0.0.0         192.168.1.1     0.0.0.0         UG    0      0        0 bond0
127.0.0.0       0.0.0.0         255.0.0.0       U     0      0        0 lo
172.31.0.0      192.168.1.171   255.255.255.0   UG    0      0        0 bond0
192.168.1.0     0.0.0.0         255.255.255.0   U     0      0        0 bond0

(A VPN client on which the route doesn't work)
[~] # ip route get 172.31.0.6
172.31.0.6 via 192.168.1.171 dev bond0  src 192.168.1.16
    cache  ipid 0x96f7

(A host that works)
[~] # ip route get 192.168.1.208
192.168.1.208 dev bond0  src 192.168.1.16
    cache  ipid 0xa016 rtt 6ms rttvar 8ms cwnd 20 reordering 83

[~] # ip addr
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 16436 qdisc noqueue state UNKNOWN
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
    inet 127.0.0.1/8 scope host lo
2: eth1: <BROADCAST,MULTICAST,SLAVE,UP,LOWER_UP> mtu 1500 qdisc mq master bond0 state UP qlen 1000
    link/ether 00:08:9b:c0:88:eb brd ff:ff:ff:ff:ff:ff
3: eth0: <BROADCAST,MULTICAST,SLAVE,UP,LOWER_UP> mtu 1500 qdisc mq master bond0 state UP qlen 1000
    link/ether 00:08:9b:c0:88:eb brd ff:ff:ff:ff:ff:ff
4: bond0: <BROADCAST,MULTICAST,MASTER,NOTRAILERS,UP,LOWER_UP> mtu 1500 qdisc noqueue state UP
    link/ether 00:08:9b:c0:88:eb brd ff:ff:ff:ff:ff:ff
    inet 192.168.1.16/24 brd 192.168.1.255 scope global bond0