That's one reason I stopped using Windows. File associations are not supposed to change by themselves. Microsoft software (and Apple's quicktime software) on Windows was notorious for "hijacking" file associations behind the users' backs. I suppose the same thing could happen on a Mac. No. Wait. The same thing did happen when I installed Office 2011 on my wife's Mac. Double clicking any file format that might be even remotely possible to associate with MS Office caused an MS Office app to launch (slowly). When her HDD crashed, I "forgot" 😉 to reinstall Office 2011 and she hasn't "reminded" me so all is well. 😎
In my mind Lotus Notes begs for it's developers to be brought before the Hague tribunal for crimes against humanity. In all of human history there isn't another piece of software that is so bloated, with so many obfuscated context sensitive preference windows, redundant nested scroll bars and menus and tool bars scattered randomly throughout the user interface. Yuck. Just yuck. I find it difficult to stomach Outlook but if I could choose between Notes and Outlook, I'd have to go with Outlook. Unfortunately many corporate IT departments force the use of Notes. I remember "back in the day" when anyone using Notes was immune to the "worm of the week" and Outlook users were shut down. But these days Outlook works well enough that it really isn't necessary to keep using Notes except for to some contractual arrangement with IBM.
Anyway, glad you got it sorted out. Association stealing is rude behavior and wastes users' time figuring out why something has suddenly changed through no fault of their own.