Sorry, got kinda stuck trying to test with a USB NIC: https://askubuntu.com/q/1212529/325336 

After disabling systemd-resolved (+ putting a public DNS IPv4 address in resolv.conf), v6 download over the external USB NIC (Apple USB Ethernet, 05ac:1402) works fine. After switching the cable back to the built-in NIC, the error appeared almost immediately, didn't even get to try the v6 download.

[ 2452.471188] br0: port 3(eth1) entered disabled state
[ 3030.723594] br0: received packet on wlan0 with own address as source address (addr:b8:27:eb:3a:ba:a2, vlan:0)
[ 3030.860954] br0: received packet on wlan0 with own address as source address (addr:b8:27:eb:3a:ba:a2, vlan:0)
[ 3037.463785] br0: received packet on wlan0 with own address as source address (addr:b8:27:eb:3a:ba:a2, vlan:0)
[ 3228.800706] asix 1-1.3:1.0 eth1: link up, 100Mbps, full-duplex, lpa 0xCDE1
[ 3228.804385] br0: port 3(eth1) entered blocking state
[ 3228.804417] br0: port 3(eth1) entered forwarding state
[ 3293.930873] ------------[ cut here ]------------
[ 3293.930958] NETDEV WATCHDOG: eth0 (lan78xx): transmit queue 0 timed out
[ 3293.931102] WARNING: CPU: 3 PID: 0 at net/sched/sch_generic.c:447 dev_watchdog+0x324/0x330
[ 3293.931109] Modules linked in: asix sit tunnel4 ip_tunnel bridge stp llc ip6table_filter ip6_tables xt_tcpudp xt_conntrack nf_conntrack nf_defrag_ipv6 nf_defrag_ipv4 iptable_filter bpfilter nls_ascii dm_multipath scsi_dh_rdac scsi_dh_emc scsi_dh_alua btsdio bluetooth ecdh_generic bcm2835_v4l2(CE) ecc bcm2835_mmal_vchiq(CE) vc_sm_cma(CE) brcmfmac v4l2_common videobuf2_vmalloc brcmutil videobuf2_memops videobuf2_v4l2 cfg80211 videobuf2_common input_leds videodev mc spidev raspberrypi_hwmon uio_pdrv_genirq uio sch_fq_codel ip_tables x_tables autofs4 btrfs zstd_compress raid10 raid456 async_raid6_recov async_memcpy async_pq async_xor async_tx xor xor_neon raid6_pq libcrc32c raid1 raid0 multipath linear hid_generic usbhid crct10dif_ce sdhci_iproc phy_generic fixed aes_neon_bs aes_neon_blk crypto_simd cryptd aes_arm64
[ 3293.931334] CPU: 3 PID: 0 Comm: swapper/3 Tainted: G         C  E     5.3.0-1018-raspi2 #20-Ubuntu
[ 3293.931339] Hardware name: Raspberry Pi 3 Model B Plus Rev 1.3 (DT)
[ 3293.931348] pstate: 60400005 (nZCv daif +PAN -UAO)
[ 3293.931356] pc : dev_watchdog+0x324/0x330
[ 3293.931363] lr : dev_watchdog+0x324/0x330
[ 3293.931366] sp : ffff00001001bd60
[ 3293.931371] x29: ffff00001001bd60 x28: 0000000000000140 
[ 3293.931380] x27: 00000000ffffffff x26: 0000000000000000 
[ 3293.931388] x25: fffff2fb79363000 x24: ffff4f7cab129018 
[ 3293.931396] x23: 0000000000000000 x22: 0000000000000001 
[ 3293.931404] x21: fffff2fb79363480 x20: ffff4f7cab407000 
[ 3293.931412] x19: 0000000000000000 x18: 0000000000000000 
[ 3293.931420] x17: ffff000010fd8218 x16: ffff4f7ca9f0f060 
[ 3293.931428] x15: fffff2fb79228510 x14: ffffffffffffffff 
[ 3293.931437] x13: 0000000000000000 x12: ffff4f7cab540000 
[ 3293.931445] x11: ffff4f7cab42d000 x10: 0000000000000000 
[ 3293.931453] x9 : 0000000000000004 x8 : 0000000000000192 
[ 3293.931461] x7 : 0000000000000000 x6 : 0000000000000001 
[ 3293.931468] x5 : 0000000000000000 x4 : 0000000000000008 
[ 3293.931476] x3 : ffff4f7caaa15750 x2 : 0000000000000004 
[ 3293.931484] x1 : 59c84c96ab783c00 x0 : 0000000000000000 
[ 3293.931492] Call trace:
[ 3293.931500]  dev_watchdog+0x324/0x330
[ 3293.931515]  call_timer_fn+0x3c/0x178
[ 3293.931525]  __run_timers.part.0+0x200/0x330
[ 3293.931533]  run_timer_softirq+0x40/0x78
[ 3293.931544]  __do_softirq+0x168/0x384
[ 3293.931554]  irq_exit+0xb0/0xe8
[ 3293.931564]  __handle_domain_irq+0x70/0xc0
[ 3293.931572]  bcm2836_arm_irqchip_handle_irq+0x74/0xe0
[ 3293.931579]  el1_irq+0x108/0x200
[ 3293.931590]  arch_cpu_idle+0x3c/0x1c8
[ 3293.931603]  default_idle_call+0x24/0x48
[ 3293.931615]  do_idle+0x210/0x2a0
[ 3293.931623]  cpu_startup_entry+0x2c/0x30
[ 3293.931636]  secondary_start_kernel+0x154/0x1c8
[ 3293.931641] ---[ end trace 356cf2f8b780723e ]---
[ 3734.263219] br0: port 3(eth1) entered disabled state
[ 3737.505916] lan78xx 1-1.1.1:1.0 eth0: kevent 0 may have been dropped
[ 3737.523763] lan78xx 1-1.1.1:1.0 eth0: kevent 0 may have been dropped
[ 3737.541622] lan78xx 1-1.1.1:1.0 eth0: kevent 0 may have been dropped
[ 3737.559099] lan78xx 1-1.1.1:1.0 eth0: kevent 0 may have been dropped
[ 3737.576635] lan78xx 1-1.1.1:1.0 eth0: kevent 0 may have been dropped
[ 3737.593932] lan78xx 1-1.1.1:1.0 eth0: kevent 0 may have been dropped
[ 3737.611136] lan78xx 1-1.1.1:1.0 eth0: kevent 0 may have been dropped
[ 3737.628048] lan78xx 1-1.1.1:1.0 eth0: kevent 0 may have been dropped
...