fwmark not applying when using AllowedIPs = 0.0.0.0/0

So when I run wg-quick, everything looks normal:

$ sudo WG_QUICK_USERSPACE_IMPLEMENTATION=boringtun WG_SUDO=1 wg-quick up ./wg0.conf
[#] boringtun wg0
BoringTun started successfully
[#] wg setconf wg0 /dev/fd/63
[#] ip -4 address add 10.0.0.3/32 dev wg0
[#] ip link set mtu 1420 up dev wg0
[#] wg set wg0 fwmark 51820
[#] ip -4 route add 0.0.0.0/0 dev wg0 table 51820
[#] ip -4 rule add not fwmark 51820 table 51820
[#] ip -4 rule add table main suppress_prefixlength 0
[#] sysctl -q net.ipv4.conf.all.src_valid_mark=1
[#] iptables-restore -n

But, using `wg show`, the fwmark option isn't there.

$ sudo wg show
interface: wg0
public key:
private key: (hidden)
listening port: 56761
### The fwmark setting is usually listed here

peer:
endpoint: :
allowed ips: 0.0.0.0/0
latest handshake: 14 seconds ago
transfer: 456 B received, 9.21 KiB sent
persistent keepalive: every 21 seconds

As a result, I am not able to use this to emerge via my endpoint. Nor am I able to access SSH on that endpoint server via its VPN IP address.

I have also set the caps:

boringtun = cap_net_admin+eip

However, if I use wireguard-go, this does work:

$ sudo wg show
interface: wg0
public key:
private key: (hidden)
listening port: 43448
fwmark: 0xca6c

peer:
endpoint: :
allowed ips: 0.0.0.0/0
transfer: 0 B received, 296 B sent
persistent keepalive: every 21 seconds

And I am able to route via my endpoint.

I am using Gentoo:

Linux gentoo 5.4.28-gentoo-x86_64 #1 SMP Mon Apr 27 14:39:46 -00 2020 x86_64 Intel(R) Core(TM) i5-3317U CPU @ 1.70GHz GenuineIntel GNU/Linux