Can bcache have non-zero dirty cache forever?

I created a Raspberry Pi based bcache on SSD with a HDD based RAID 1 array. After populating the RAID with few TB of content, bcache showed 10% dirty cache. That would be expected, since I have /sys/block/bcache0/bcache/cache_mode set to writeback. However, it stays at 10% indefinitely. The device is running for days without any activity. I even tried to force the cache flush by

echo 0 > /sys/block/bcache0/bcache/writeback_percent

Which was properly set, but no disk activity started, as evidenced by iostat, the dirty cache still remains at 10%. Does this mean there's something wrong with bcache? Should I worry, or is there any explanation for this behavior?