When using the mender-partuuid feature, MENDER_STORAGE_DEVICE mustn't be used and Mender also warns about doing that: https://github.com/mendersoftware/meta-mender/blob/f1c44f1a1a7034debae92fd90f1220106526ef71/meta-mender-core/classes/mender-setup.bbclass#L331
However, the mender-growfs-data feature still relies on MENDER_STORAGE_DEVICE exclusively: https://github.com/mendersoftware/meta-mender/blob/f1c44f1a1a7034debae92fd90f1220106526ef71/meta-mender-core/recipes-mender/mender-client/files/mender-client-resize-data-part.sh.in#L7
Currently, this has the effect that our SSD-based device running Yocto/Mender on /dev/sda tries to resize a partition on the bogus storage device /dev/mmcblk0 on every boot.
This may turn into a real problem once an SD card is inserted into the slot and it is manipulated for no reason.
A proper fix would probably determine the underlying storage device from the currently mounted root partition.