OK, managed to reproduce this:

[ 3913.025219] VFAT options:  check=s
[ 3913.025268] Stress test:   /home/ubuntu/autotest/client/tmp/ubuntu_vfat_stress/src/stress-ng/stress-ng --verify --times --metrics-brief --syslog --keep-name -t 10s --hdd 2 --hdd-opts sync,wr-rnd,rd-rnd,fadv-willneed,fadv-rnd --lockf 2 --seek 2 --aio 2 --aio-requests 32 --dentry 2 --dir 2 --dentry-order stride --fallocate 2 --fstat 2 --dentries 100 --lease 2 --open 2 --rename 2 --hdd-bytes 4M --fallocate-bytes 4M --chdir 2 --rename 2 --hdd-write-size 512
[ 3913.025287] Mount point:   /mnt/vfat-test-75202
[ 3913.025760] Started, PID 74908

Nov 29 12:53:00 baltar kernel: [ 4110.095234] INFO: task kworker/u321:3:80990 blocked for more than 120 seconds.
Nov 29 12:53:00 baltar kernel: [ 4110.095309]       Not tainted 4.15.0-39-generic #42-Ubuntu
Nov 29 12:53:00 baltar kernel: [ 4110.095345] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
Nov 29 12:53:00 baltar kernel: [ 4110.095398] kworker/u321:3  D    0 80990      2 0x00000800
Nov 29 12:53:00 baltar kernel: [ 4110.095406] Workqueue: writeback wb_workfn (flush-7:0)
Nov 29 12:53:00 baltar kernel: [ 4110.095408] Call Trace:
Nov 29 12:53:00 baltar kernel: [ 4110.095411] [c000000feb7b6af0] [c000000feb7b6b60] 0xc000000feb7b6b60 (unreliable)
Nov 29 12:53:00 baltar kernel: [ 4110.095416] [c000000feb7b6cc0] [c00000000001c1d0] __switch_to+0x2a0/0x4d0
Nov 29 12:53:00 baltar kernel: [ 4110.095419] [c000000feb7b6d20] [c000000000cfd2e4] __schedule+0x2a4/0xaf0
Nov 29 12:53:00 baltar kernel: [ 4110.095423] [c000000feb7b6df0] [c000000000cfdb70] schedule+0x40/0xc0
Nov 29 12:53:00 baltar kernel: [ 4110.095426] [c000000feb7b6e10] [c000000000152e7c] io_schedule+0x2c/0x50
Nov 29 12:53:00 baltar kernel: [ 4110.095429] [c000000feb7b6e40] [c0000000006d1e04] wbt_wait+0x484/0x4e0
Nov 29 12:53:00 baltar kernel: [ 4110.095432] [c000000feb7b6ee0] [c000000000698e74] blk_mq_make_request+0x104/0x6e0
Nov 29 12:53:00 baltar kernel: [ 4110.095436] [c000000feb7b6f80] [c000000000686a34] generic_make_request+0x124/0x380
Nov 29 12:53:00 baltar kernel: [ 4110.095439] [c000000feb7b6ff0] [c000000000686d4c] submit_bio+0xbc/0x1d0
Nov 29 12:53:00 baltar kernel: [ 4110.095442] [c000000feb7b70a0] [c00000000042de2c] submit_bh_wbc+0x1dc/0x240
Nov 29 12:53:00 baltar kernel: [ 4110.095445] [c000000feb7b70f0] [c00000000042e108] __block_write_full_page+0x278/0x570
Nov 29 12:53:00 baltar kernel: [ 4110.095448] [c000000feb7b7190] [c00000000054ac6c] fat_writepage+0x2c/0x40
Nov 29 12:53:00 baltar kernel: [ 4110.095451] [c000000feb7b71b0] [c00000000043bdf0] __mpage_writepage+0x170/0x7b0
Nov 29 12:53:00 baltar kernel: [ 4110.095455] [c000000feb7b76e0] [c0000000002f2d9c] write_cache_pages+0x25c/0x590
Nov 29 12:53:00 baltar kernel: [ 4110.095457] [c000000feb7b7820] [c00000000043bac8] mpage_writepages+0x78/0x150
Nov 29 12:53:00 baltar kernel: [ 4110.095460] [c000000feb7b78b0] [c00000000054abe8] fat_writepages+0x28/0x40
Nov 29 12:53:00 baltar kernel: [ 4110.095463] [c000000feb7b78d0] [c0000000002f5f1c] do_writepages+0x4c/0x130
Nov 29 12:53:00 baltar kernel: [ 4110.095465] [c000000feb7b7940] [c00000000041eff0] __writeback_single_inode+0x70/0x570
Nov 29 12:53:00 baltar kernel: [ 4110.095468] [c000000feb7b79a0] [c00000000041fb8c] writeback_sb_inodes+0x26c/0x5d0
Nov 29 12:53:00 baltar kernel: [ 4110.095470] [c000000feb7b7ab0] [c00000000042024c] wb_writeback+0x13c/0x460
Nov 29 12:53:00 baltar kernel: [ 4110.095472] [c000000feb7b7b80] [c000000000420f20] wb_workfn+0x100/0x5e0
Nov 29 12:53:00 baltar kernel: [ 4110.095475] [c000000feb7b7c90] [c000000000134bd8] process_one_work+0x298/0x5a0
Nov 29 12:53:00 baltar kernel: [ 4110.095478] [c000000feb7b7d20] [c000000000134f78] worker_thread+0x98/0x630
Nov 29 12:53:00 baltar kernel: [ 4110.095480] [c000000feb7b7dc0] [c00000000013db48] kthread+0x1a8/0x1b0
Nov 29 12:53:00 baltar kernel: [ 4110.095484] [c000000feb7b7e30] [c00000000000b658] ret_from_kernel_thread+0x5c/0x84
Nov 29 12:53:00 baltar kernel: [ 4110.095487] INFO: task stress-ng:74924 blocked for more than 120 seconds.
Nov 29 12:53:00 baltar kernel: [ 4110.095531]       Not tainted 4.15.0-39-generic #42-Ubuntu