Strange NaN area in Back-Geocoding result

Hello everyone,

Here is a problem I found in my result, some area become nodata in slave image, while the dem I input is correct. There is some figure:

The master image 20180416, east coast of Taiwan:

The slave image 20150619, there are several triangle look nodata area:

DEM data I input in same area, elevation seems no problem:

It seems Back-Geocoding operator have some bug like that?
I also mention about another similar problem in this thread:

Does anyone encounter to this problem with your own external DEM?

So you used your own DEM?
Please try once with SRTM 1Sec (Auto Download) to check if the error persists. It should not affect the coregistration quality to use it.