I experience 100% packet loss, floppy interface state. I'm only able to use the interface when configured to media 10baseT/UTP. Below are my system details:

# dmesg | grep rgephy0
rgephy0: <RTL8251/8153 1000BASE-T media interface> PHY 1 on miibus0
rgephy0:  none, 10baseT, 10baseT-FDX, 10baseT-FDX-flow, 100baseTX, 100baseTX-FDX, 100baseTX-FDX-flow, 1000baseT-FDX, 1000baseT-FDX-master, 1000baseT-FDX-flow, 1000baseT-FDX-flow-master, auto, auto-flow

# pciconf -lvcev re0
re0@pci0:3:0:0: class=0x020000 card=0x86771043 chip=0x816810ec rev=0x15 hdr=0x00
    vendor     = 'Realtek Semiconductor Co., Ltd.'
    device     = 'RTL8111/8168/8411 PCI Express Gigabit Ethernet Controller'
    class      = network
    subclass   = ethernet
    cap 01[40] = powerspec 3  supports D0 D1 D2 D3  current D0
    cap 05[50] = MSI supports 1 message, 64 bit 
    cap 10[70] = PCI-Express 2 endpoint MSI 1 max data 128(128) RO
                 link x1(x1) speed 2.5(2.5) ASPM disabled(L0s/L1)
    cap 11[b0] = MSI-X supports 4 messages, enabled
                 Table in map 0x20[0x0], PBA in map 0x20[0x800]
    ecap 0001[100] = AER 2 0 fatal 0 non-fatal 2 corrected
    ecap 0002[140] = VC 1 max VC0
    ecap 0003[160] = Serial 1 01000000684ce000
    ecap 0018[170] = LTR 1
    ecap 001e[178] = unknown 1
     Corrected = Receiver Error
                 Bad DLLP

# ifconfig re0 media 100baseTX mediaopt full-duplex
# ifconfig re0
re0: flags=8843<UP,BROADCAST,RUNNING,SIMPLEX,MULTICAST> metric 0 mtu 1500
        options=8009b<RXCSUM,TXCSUM,VLAN_MTU,VLAN_HWTAGGING,VLAN_HWCSUM,LINKSTATE>
        ether b0:6e:bf:81:cd:56
        inet 192.168.0.20 netmask 0xffffff00 broadcast 192.168.0.255
        media: Ethernet 100baseTX <full-duplex>
        status: active
        nd6 options=29<PERFORMNUD,IFDISABLED,AUTO_LINKLOCAL>
root@viper:~ # ping -q -c 10 192.168.0.1
PING 192.168.0.1 (192.168.0.1): 56 data bytes

--- 192.168.0.1 ping statistics ---
10 packets transmitted, 0 packets received, 100.0% packet loss

# ifconfig re0 media 100baseTX mediaopt full-duplex
# ifconfig re0
re0: flags=8843<UP,BROADCAST,RUNNING,SIMPLEX,MULTICAST> metric 0 mtu 1500
        options=8009b<RXCSUM,TXCSUM,VLAN_MTU,VLAN_HWTAGGING,VLAN_HWCSUM,LINKSTATE>
        ether b0:6e:bf:81:cd:56
        inet 192.168.0.20 netmask 0xffffff00 broadcast 192.168.0.255
        media: Ethernet 100baseTX <full-duplex>
        status: active
        nd6 options=29<PERFORMNUD,IFDISABLED,AUTO_LINKLOCAL>
# ping -q -c 10 192.168.0.1
PING 192.168.0.1 (192.168.0.1): 56 data bytes

--- 192.168.0.1 ping statistics ---
10 packets transmitted, 0 packets received, 100.0% packet loss

# dmesg | grep rgephy0
rgephy0: <RTL8251/8153 1000BASE-T media interface> PHY 1 on miibus0
rgephy0:  none, 10baseT, 10baseT-FDX, 10baseT-FDX-flow, 100baseTX, 100baseTX-FDX, 100baseTX-FDX-flow, 1000baseT-FDX, 1000baseT-FDX-master, 1000baseT-FDX-flow, 1000baseT-FDX-flow-master, auto, auto-flow
# netstat -ni
Name    Mtu Network       Address              Ipkts Ierrs Idrop    Opkts Oerrs  Coll
re0    1500 <Link#1>      b0:6e:bf:81:cd:56      603     0     0      681     0     0
re0       - 192.168.0.0/2 192.168.0.20           538     -     -      624     -     -


The following bug IDs seem relevant, 186401 and 193743; however, I am using FreeBSD/12.1-RELEASE-p1 so it should be patched -- right?