Why does `chattr +e <file>` fail with no space left on the device when there's plenty?

I have a problem. chattr +e <file> does not work, if the file does not have the +e attr set on it, or if it's removed and re-added.

[root@ip-203-0-113-13 log]# df -Th
Filesystem     Type      Size  Used Avail Use% Mounted on
devtmpfs       devtmpfs  3.8G     0  3.8G   0% /dev
tmpfs          tmpfs     3.8G     0  3.8G   0% /dev/shm
tmpfs          tmpfs     3.8G  388K  3.8G   1% /run
tmpfs          tmpfs     3.8G     0  3.8G   0% /sys/fs/cgroup
/dev/nvme0n1p1 ext4       30G  1.2G   28G   5% /
tmpfs          tmpfs     763M     0  763M   0% /run/user/1000
[root@ip-203-0-113-13 log]# df -ih
Filesystem     Inodes IUsed IFree IUse% Mounted on
devtmpfs         950K   343  949K    1% /dev
tmpfs            953K     1  953K    1% /dev/shm
tmpfs            953K   389  953K    1% /run
tmpfs            953K    17  953K    1% /sys/fs/cgroup
/dev/nvme0n1p1   1.9M   97K  1.8M    6% /
tmpfs            953K     5  953K    1% /run/user/1000
[root@ip-203-0-113-13 log]# find /tmp -type f | wc -l
6126
[root@ip-203-0-113-13 tmp]# touch bar
[root@ip-203-0-113-13 tmp]# chattr -e bar
[root@ip-203-0-113-13 tmp]# chattr +e bar
chattr: No space left on device while setting flags on bar

Waaaaat?

There is plenty of space on the device and plenty of inodes free, so why is chattr +e failing? If it makes a difference, I'm seeing this failure on Fedora 28 on EC2. I'm not seeing on SUSE 42 on EC2 (yet?)

Update:

I ran strace on the command. I'm not sure if it's helpful, but it seems interesting...