There is no reason to provide exact reason since IRCC will have it in the system and they will do a FOSS check on your application anyway. The important thing is to mention that your visa was refused, and in the reason you can mention the you no longer remember why. Also, in most refusals the reason is generic.
To know the exact reason, for Canada, you can request the GCMS notes for your refused application. You can also request the copy of your entire application with the supporting materials, if you do not remember what you wrote in the application or document you provided.
IRCC does pick up previous applications to verify details, and it there is any contradiction on material facts, that can jeopardize your current application. Not trying to scare you, but an applicant is responsible to answer all questions truthfully.
Thank you so much for taking the time to respond.