Under some circumstances external graphics become hard to select neither by clicking on it, nor by dragging near drawing. However, once the object is moved, the issue disappears until next time, until document is reopened.
https://0x0.st/HN2l.lha

Confirmed, there seems to be some issue with the current selection box if you click on it. It only works once you use a large lasso to select the graphics when the object tool is active, you move it a bit and then the internal (selection) bounding box seems to get updated. If you save and reload, it is back to hardly selectable. There seems to be a connection to keeping it as external file indeed.