@mrmikel - I still can't find or see a way to break it, it behaves totally as expected for us. Without more detail or seeing it for ourselves I don't have anything I can suggest.
The code is relatively simple - every time you close the Preferences window, it will *completely overwrite* the currently selected word list file with whatever the words are that are displayed in the word list (assuming you made a change of some kind on that Spelling tab of Preferences).
Once those words are written to the file, it reloads the dictionary in Sigil based on the new contents of that file and that is what will be used when words are underlined in the open tabs.
When you add a word using a right-click in Code View, it immediately *appends* to that same file the word you right-clicked on.
Then when you go back into Preferences, it opens that same words list file, reads the words from it and that is what gets displayed to you. And hence the cycle could begin again...
As I mentioned above what you need to do is keep the "default" file open in Notepad++ and make a list of *exactly* what actions you are clicking on (not "summarised" - "add three at once" doesn't tell us whether you added the words using code view or on the Preferences dialog etc). And equally make note every time Noteapd++ tells you the "default" file has changed at the associated points and what it now contains. That may elicit more hints as to what is happening in combination with the information I have stated above.
And no it is not possible to have multiple dictionaries loaded at once currently, in theory it could be made to do so in a future version though it would slow down your Sigil startup time for sure.