As per several reddit discussions, the ads seem to be powered by Magazine Unlock, a feature available on Huawei phones that automatically replaces the lock screen wallpaper after each unlock. But instead of setting a new lock screen background, Magazine Unlock sometime displays an ad for hotel reservation website booking.com.
Neither Huawei nor Booking offered any comments on this ad push, albeit the official account of Huawei Germany released an apology and recommended users who want to block the ads to disable Magazine Unlock.
I'm sorry that you think we want to advertise. You can deactivate the magazine at any time. Just wipe it from bottom to top and click on the gearwheel symbol.
Best regards, Tolga from HUAWEI Mobile
Source: Huawei Puts Ads on Lock Screens Because It Doesn’t Already Have Enough Problems