Did you ensure that the thumbnail sizes are the same on the two devices as was discussed in
https://www.mobileread.com/forums/sho....php?t=225172? It should be sufficient to set the thumbnail size to "large" on both devices, connect and sync each one, then set it to whatever you really want.
The only way to remove the requirement that the thumbnails be the same size is to create a separate metadata cache per device. I am still working on that.