After making a number of edits then noticing that the explanation I had put in the "Explain your changes" field was missing from the change list, I've determined that if you copy text then paste it into the "Explain your changes" field, it is ignored.
Has anyone else seen this? I'm using Firefox on an iPad if that's relevant. This situation can occur if you're making a series of changes to several pages with the same explanation.
A workaround is to type a character at the end of the pasted-in explanation then delete it again. This seems to alert the software that there is text in the field.