I know what you're talking about, but the fare brand displayed part of an overall murky naming scheme, it really sucks how confusing it is. This happens when booking through non-AC channels. If a Business Class Flexible reward was booked through Aeroplan (the fare brand eligible for YYZ SS), it would say: "Business Class - Flexible Reward". Note the hyphen and the word "reward". Also note the capitalization because it's an actual brand of fares that you can purchase (your other option would be Business Class Lowest)

Because you didn't book through Aeroplan, it's simply showing a slightly different naming convention to refer to your unrestricted business class award, that Air Canada acknowledges that they will transport you on.

A better example to reinforce my point would be through a recent experience where I booked an economy class ticket via Expedia. You might know that Air Canada has Flex (a restricted economy fare) and Latitude (an unrestricted economy fare) fare brands when booking directly through AC. When you book a full fare (Y/B) through an online travel agent, and look up the reservation on AC's site, it'll show up as "Economy (flexible)" (note no hyphen, parentheses, no Economy "Class", and no proper noun for fare brand) because the website doesn't infer the fare brand at this point when showing the fare you booked...probably because I wasn't presented with the brands when booking through non-AC

Your business class ticket happens to be a flexible ticket that UA sold you, but it's not the Business Class Flexible fare brand that AC sells you allowing you entry

Of course, all this is moot if you have a paid business class fare to an eligible Signature Suite destination, regardless of who issued the ticket. Air Canada can obviously detect that, and you deep down in your heart would know you'd be able to enter