When copying and pasting from MS Word, it brings along a lot of extra code (I call it legacy code) that causes all kinds of havoc. Sometimes it has no affect; other times it can cause weird things to happen. This legacy code is used by MS Word for formatting but when brought into Moodle, the HTML editor can't understand it. You can see an example of legacy code by copying and pasting from MS Word into Moodle and then toggling to the HTML source in Moodle.
As an alternative, copying the text into a simple text editor (Notepad on Windows, TextEdit on Mac) and that will remove the legacy code. Then copying that text into Moodle and re-format it as needed with Moodle's HTML editor.