ah yes, I know about that trick moving to right then left however sometimes you can't do it if the last image is on the first column.
Focusing in the selection problem I forgot that the image displayed in the browser isn't correct when you try to select the 5th image which I'm pretty sure is a bug.
All I did here is select the first image then press 4 times right arrow while holding shift. You can see the image in the browser corresponds to the 3rd image while it should show the 5th image, because it was the last one selected.
As far as I have tested the deselection (bug?) only happens if you use right arrow at the last column, if you tried to do it at the first column using the left arrow it doesn't deselect a thumbnail.
If you select an image on the last column hold shift then press right arrow then left arrow you end up with 2 images selected (1 if you do it on a different column) then if you press left arrow again you end up with 2 images selected (same as if you do it on a different column)