Routing table messed up at boot

[arnoldus]

I configured a Raspberry 1 (with newest and up to date Raspbian) to be a SSH server. It worked nicely in my home LAN.
When I put it in my office LAN, I could not access it from WAN. It can see other LAN-addresses and get pinged, but nothing WAN.

After troubleshooting the router, firewalls, etc, I found that the routing table is messed up.
I don't understand why.

/etc/network/interfaces

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source-directory /etc/network/interfaces.d

auto lo
iface lo inet loopback

auto eth0
iface eth0 inet dhcp

allow-hotplug wlan0
oface wlan0 inet manual
wpa-conf /etc/wpa_supplicant/wpa_supplicant.conf

Route

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Kernel IP routing table
Destination	Gateway		Genmask	Flags	Metric	Ref	Use	Iface
10.8.0.0	192.168.100.94	255.255.255.0	UG	0	0	eth0
10.8.0.0	192.168.100.94	255.255.255.0	UG	202	0	eth0
192.168.100.0	0.0.0.0	255.255.255.0	U	0	0	eth0
192.168.100.0	0.0.0.0.	255.255.255.0	U	202	0	eth0

When I do

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sudo ip route add default via 192.168.100.254

it seems to be working. But that does not persist after reboot.

There is another Raspberry 1 in that LAN, it does not have any issues.

192.168.100.254 is my router
192.168.100.94 is my VPN gateway

What is going on? How do I solve it properly?