Bug(?) transferring a tif from C1 Express to LR 5.7

When Nikon discontinued Capture NX I moved to LR  and have built up a catalog of all my images in LR. I've now moved over completely to Fuji. Recently I started using C1 Express to process the raf file. I use LR for cataloguing sooc jpegs and then process selected rafs in C1 Express. I transfer these processed files as tifs from C1 to LR for printing, etc. This has worked without a hitch for maybe 60 or 70 X-T10 rafs.

This afternoon I put X-100S files through the same process. The first file transferred as expected from C1 to LR. However the second image was surprisingly about two stops lighter in LR than in C1. I checked the tif file output by C1 in a general file viewer and in the viewer it was at the expected density.

So the problem is presumably with LR, which appears to be applying a ~ +2EV adjustment to the tif file when importing it.

I don't understand why this would happen and would value any input!

Possible complicating factors:

1 There are raf and DNG (via IXT) versions of this file already in the LR catalog. Is there some interaction in LR between files with the same name though different extensions? (Seems unlikely.)

2 These are the first files I've transferred from C1 to LR since fitting a (larger) clone of the system disk. Is this LR error a consequence of the disk cloning process? (Seems unlikely. I've picked up no other anomalies with the cloned disk, and LR, C1, and other programs seem to run normally.)

Any suggestions on what might be happening or how to deal with it would be greatly appreciated!

John.