Yes this bug has been there from day one with the guides. If the guide software would just kindly forget tags that were deleted, you could delete all the offending tags and start again. But it remembers them, and corrects your new entries back to whatever you first entered.
(I document this in the “Bugs and limitations” section of my tutorial https://forum.inaturalist.org/t/a-guide-to-inaturalist-guides/950)
The only work around that I know is forget about your original “petal color” and “Petal color” tags - delete them from every page, and carefully add a new version eg “Color of petals” or “Petal colour” if you like British English.
This bug is really annoying if some of the images you upload have tags in their meta data with the wrong case version of your tags.
–Tony