It is likely that a lcd screen was used for the repair. Apple is a little overzealous with anything not directly from Apple but he CPOs lately have been using off spec components for repairs for resale. There is no “fix” for this. It’s what you got from Koodo. I would return it but if you’re ok with it, then it’s up to you on what you want to do. 

OLED versus LCD, there is a noticeable difference. OLED screens have richer colours and deeper blacks that you cannot get with an LCD screen. Power saving is built into OLED displays because the screen turns off pixels when black colours are displayed. Switching to dark mode also helps to reduce power consumption. 

An LCD screen tends to use more power because it uses a white light source to illuminate the entire back of the panel. Dark mode has no effect in terms of power saving and dark mode may not be selectable.

The iPhone 11 Pro Max originally shipped with a Super Retina XDR, 6.5‑inch (diagonal) all‑screen OLED Multi‑Touch, HDR display. I would return the iPhone. Unfortunately, with CPO iPhones listed in Great or Very Good condition, you may receive another device repaired with non-genuine parts. If available, you may have better luck with an iPhone in Like-New condition.