I get an unintended file name truncation when opening a file named CAB_20161129_0900-Pano.dng in PS and then re-saving it. PS immediately renames it to CAB_20161129_.tif, dropping the file sequence number and other text. It matters not whether the file saving convention is for a .tif or a .psd.
The file name is changed IN Photoshop immediately as the FILE>SAVE command is issued.
However, if the file's name is just the CAB_20161129_0900.dng, (i.e. does not have the added Pano text in it) then the file name is saved correctly as CAB_3016_0900.tif (or .psd as may be the case).
This happens regularly despite care in setting up the file naming conventions, etc.
PS CC (2017), LR CC (2015) all up to date.
MacPro (2013), El Capitan.
You need to set Custom settings for edit externally file name. Open Lightroom > Edit menu > Preferences > External Editing > Check the Template( Last section )