I’ve been having the same thing happen, i’ve gotten firefox, kernels and nvidia driver updates when there’s a testing update even though i’m on stable and there hasn’t been a stable update in a week+
$ sudo pacman -Syuu
:: Synchronizing package databases...
core is up to date
extra is up to date
community is up to date
multilib is up to date
:: Starting full system upgrade...
resolving dependencies...
warning: cannot resolve "nvidia-450xx-utils=450.80.02", a dependency of "linux54-nvidia-450xx"
warning: cannot resolve "nvidia-450xx-utils=450.80.02", a dependency of "linux58-nvidia-450xx"
:: The following packages cannot be upgraded due to unresolvable dependencies:
linux54-nvidia-450xx linux58-nvidia-450xx
:: Do you want to skip the above packages for this upgrade? [y/N] y
looking for conflicting packages...
Packages (16) amd-ucode-20201005.r1732.58d41d0-1 linux-firmware-20201005.r1732.58d41d0-1 linux54-5.4.72-1 linux54-headers-5.4.72-1
linux58-5.8.16-2 linux58-headers-5.8.16-2 manjaro-release-20.1.2-1 mhwd-db-0.6.5-4 mhwd-nvidia-450xx-450.80.02-1
mhwd-nvidia-455xx-455.28-2 pamac-cli-9.5.12-1 pamac-common-9.5.12-1 pamac-flatpak-plugin-9.5.12-1 pamac-gtk-9.5.12-1
pamac-snap-plugin-9.5.12-1 pamac-tray-appindicator-9.5.12-1
Total Download Size: 315.81 MiB
Total Installed Size: 960.19 MiB
Net Upgrade Size: 0.95 MiB
$ pacman-mirrors -G
stable