I've found that the best way to get this to work every single time is to open the Import window in Lightroom and then drag the images into that. In the past I've also found that if you aren't dragging into the import module and just dragging into the main LR window, Lighroom's behavior can also vary depending on the kind of file and if there is an associated XMP file (for example, if the items dragged were not all of one file type, like a combination of CR2 and CR3 raw files seemed to trip it up). I'm not sure why doing things a second time would matter, but as far as I know, but if you're not changing the selection of items on the contact sheet, then Photo Mechanic isn't doing anything different with respect to what happens when you do a drag and drop.