When you are doing a spellcheck or correcting words using the context menu, there is a misfeature that makes things much harder than they need to be. When you select Add in either the spellcheck dialog or the context menu, rather than adding the word, LibreOffice then asks which dictionary you want to add the word to. It does this even if you have only one user-defined dictionary, which makes no sense.
This is well intentioned but highly inconvenient. In the spellcheck dialog box, it makes it nearly impossible to quickly add words with keyboard shortcuts and slows things down a lot with the mouse. In the context menu, it again slows down keyboard shortcuts and adds extra mouseclicks and complexity.
A better solution to this would be to add a “this dictionary gets user-defined words” in LibreOffice Options → Language Settings → Writing Aids, perhaps with a way to easily move words from one dictionary to another. But in the short term, is there a way to turn this misfeature off, so that clicking Add actually adds the word to the dictionary?