Images in Level Strip have disappeared while merging TLV levels and turned blank!

[eunanimi]

Hello, while merging a couple of levels, the images in Level strip "I_merged_1" have completely disappeared! I see nothing in the canvas nor in the level strip!

They have line art and colors in them. Is there a way to bring them back?  

In the drawings folder,  the files show up as below, but when opened in OT it is invisible (only blank frames).