My # (APPENDIX) Appendix {-} which was being treated previously as an unnumbered header:

is no longer being treated as such:

I am using the same (newest) version of bookdown. I'm wondering if something changed in gitbook?
Another change from before is that I specified split_by: chapter+number in _output.yml so previously my output files were:
1-main.html
A-supplemental.html
and now they are:
main.html
appendix-appendix.html
supplemental.html
The appendix-appendix.html is definitely erroneous as it should be an unnumbered header. Is anyone else noticing similar behaviour?