Thanks for the speedy fix!
On further testing this issue reproduces under 0.18.7 too, suggesting that the fix to https://sourceforge.net/p/ruamel-yaml/tickets/507/ maaay be related based on the changelog entries. The issue does not reproduce under 0.18.6.
Zero (0) keys don't round-trip in 0.18.8 under YAML 1.1