Due to the clean new look the BMS login page now has, your customized logo (if configured) will now be displayed on a white background on the login page.
This new change was introduced in "BMS 4.0.21" as part of a new UI we now have in place.
You may want to upload a dark version of your logo as black or charcoal gray is recommended because it will stand out on both the top bar, and the login page. As well as having the black being consistent with the menu bar in BMS. Example: