@theluthier You started creating the "Hair Length Texture" with the "Image Texture" linked to the "Texture Paint Slot" named "feathersBody_Density.png"(*).
(*) Despite the ".png" it's not the "Image File" itself but the "Texture Paint Slot Name" that just copies the "Image File Name" when this image is created or loaded into Blender for the first time!
Screenshot at 11:55
By saving the "Hair Length Texture" with "Save As" (11:45) you linked that texture to the currently selected "Texture Paint Slot" which was at that time "feathersBody_Density.png" (displayed in the bottom left corner of the screenshot above) and which you shortly after renamed to "feathersBodyB_Length".
You should have saved the file instead with "Save as Copy" in order to avoid that linking. Please read my post"Texture Paint Slots Naming and Image Selection" (especially the last paragraph) under this video which shows that this behaviour is still the same in Blender 2.90.