it’s a photo2 behaviour: while inserting a CGI overlay
rendered to HDR/32-bit openEXR documents as a layer or a group of layers into a 16-bit document containing a digiphoto
levels (or exposure) of the inserted layer(s) shifts to dark.
no idea, if i miss a setting (which should be the same in photo1 and photo2). in photo1 inserting these layer(s) gives the expected result.