There's also WARN_ON backtrace from the kernel in the attached rpi3 log:

[  656.853716] ------------[ cut here ]------------
[  656.858465] WARNING: CPU: 1 PID: 1 at arch/arm/mm/dma-mapping.c:515 pool_allocator_alloc+0xb4/0xb8
[  656.867585] coherent pool not initialised!
[  656.871768] Modules linked in: vc4 drm_kms_helper snd_soc_core snd_compress ac97_bus snd_pcm_dmaengine snd_pcm snd_timer snd fb_sys_fops syscopyarea sysfillrect sysimgblt crypto_simd cryptd linear multipath raid0 raid1 raid456 async_raid6_recov async_memcpy async_pq async_xor async_tx raid10 btrfs libcrc32c xor xor_neon zstd_compress crc32_arm_ce spidev raid6_pq drm sch_fq_codel uio_pdrv_genirq uio rpivid_mem raspberrypi_hwmon bcm2835_v4l2(CE) videobuf2_vmalloc videobuf2_memops bcm2835_mmal_vchiq(CE) vc_sm_cma(CE) videobuf2_v4l2 videobuf2_common videodev mc brcmfmac cfg80211 brcmutil input_leds btsdio bluetooth ecdh_generic ecc scsi_dh_alua scsi_dh_emc scsi_dh_rdac dm_multipath phy_generic sha256_arm echainiv aes_arm dm_crypt algif_skcipher af_alg virtio_blk virtio_scsi nls_iso8859_1 usb_storage libahci hid_generic usbhid nvme nvme_core i2c_bcm2708 ip_tables x_tables autofs4
[  656.950794] CPU: 1 PID: 1 Comm: shutdown Tainted: G        WC  E     5.4.0-1011-raspi #11-Ubuntu
[  656.959705] Hardware name: BCM2835
[  656.963152] Backtrace:
[  656.965646] [<c0239048>] (dump_backtrace) from [<c02393a8>] (show_stack+0x20/0x24)
[  656.973330]  r7:ffffffff r6:00000000 r5:600b0013 r4:c148fe14
[  656.979080] [<c0239388>] (show_stack) from [<c0c36100>] (dump_stack+0xd4/0x118)
[  656.986503] [<c0c3602c>] (dump_stack) from [<c0253be8>] (__warn+0xe0/0x108)
[  656.993571]  r10:00000000 r9:dfe56740 r8:00000009 r7:00000203 r6:c0243464 r5:00000009
[  657.001514]  r4:c1058a24 r3:00000000
[  657.005146] [<c0253b08>] (__warn) from [<c0254008>] (warn_slowpath_fmt+0xa8/0xcc)
[  657.012740]  r7:c0243464 r6:00000203 r5:c1058a24 r4:c1058a04
[  657.018489] [<c0253f64>] (warn_slowpath_fmt) from [<c0243464>] (pool_allocator_alloc+0xb4/0xb8)
[  657.027319]  r8:00000707 r7:00000000 r6:c11360b4 r5:e611fc64 r4:00000000
[  657.034127] [<c02433b0>] (pool_allocator_alloc) from [<c0244114>] (__dma_alloc+0x158/0x2f0)
[  657.042601]  r6:ffffffff r5:e6340810 r4:00000a20
[  657.047294] [<c0243fbc>] (__dma_alloc) from [<c024434c>] (arm_dma_alloc+0x54/0x5c)
[  657.054979]  r10:dfe56780 r9:00000000 r8:00000000 r7:e611fd70 r6:00000000 r5:00000000
[  657.062922]  r4:00c00000
[  657.065498] [<c02442f8>] (arm_dma_alloc) from [<c02e08e0>] (dma_direct_alloc+0x48/0x6c)
[  657.073618]  r5:00000a20 r4:e611fd70
[  657.077251] [<c02e0898>] (dma_direct_alloc) from [<c02df114>] (dma_alloc_attrs+0x94/0x11c)
[  657.085637]  r6:00001000 r5:00000a20 r4:e6340810
[  657.090332] [<c02df080>] (dma_alloc_attrs) from [<c0a31064>] (rpi_firmware_property_list+0x6c/0x180)
[  657.099602]  r8:00001000 r7:e63176c0 r6:00000018 r5:0000000c r4:dfe56780
[  657.106409] [<c0a30ff8>] (rpi_firmware_property_list) from [<c0a311e8>] (rpi_firmware_property+0x70/0x118)
[  657.116210]  r10:dfe5678c r9:00030048 r8:e63176c0 r7:00000000 r6:00000000 r5:0000000c
[  657.124152]  r4:dfe56780
[  657.126728] [<c0a31178>] (rpi_firmware_property) from [<c0a31310>] (rpi_firmware_notify_reboot+0x48/0x50)
[  657.136441]  r10:00000058 r9:e611e000 r8:c147f41c r7:00000000 r6:00000001 r5:00000000
[  657.144385]  r4:fffffffe r3:00000000
[  657.148020] [<c0a312c8>] (rpi_firmware_notify_reboot) from [<c027b79c>] (notifier_call_chain+0x58/0x94)
[  657.157560] [<c027b744>] (notifier_call_chain) from [<c027c0bc>] (blocking_notifier_call_chain+0x60/0x7c)
[  657.167272]  r9:e611e000 r8:fee1dead r7:c14167b8 r6:00000000 r5:00000001 r4:c141664c
[  657.175138] [<c027c05c>] (blocking_notifier_call_chain) from [<c027d648>] (kernel_restart_prepare+0x28/0x48)
[  657.185110]  r6:a2e2ee00 r5:00000000 r4:00000000
[  657.189803] [<c027d620>] (kernel_restart_prepare) from [<c027d758>] (kernel_restart+0x1c/0x60)
[  657.198546] [<c027d73c>] (kernel_restart) from [<c027daf4>] (__do_sys_reboot+0x20c/0x234)
[  657.206843]  r5:00000000 r4:01234567
[  657.210473] [<c027d8e8>] (__do_sys_reboot) from [<c027db8c>] (sys_reboot+0x18/0x1c)
[  657.218245]  r8:c02012e4 r7:00000058 r6:bee93b94 r5:00000001 r4:01234567
[  657.225049] [<c027db74>] (sys_reboot) from [<c0201120>] (ret_fast_syscall+0x0/0x28)
[  657.232816] Exception stack(0xe611ffa8 to 0xe611fff0)
[  657.237944] ffa0:                   01234567 00000001 fee1dead 28121969 01234567 a2e2ee00
[  657.246246] ffc0: 01234567 00000001 bee93b94 00000058 bee93b98 bee93b94 fffff000 00000001
[  657.254545] ffe0: 00000058 bee93b0c b6e88d65 b6e07be6
[  657.259732] ---[ end trace 3942ee25c15e324e ]---