System information (version)

• OpenCV => 4.5.4
• Operating System / Platform => Ubuntu 18.04.3 LT
• Compiler =>gcc

Detailed description

I test sift by asift.cpp ,it runs normally.
then i add mask regin, I found mask regin still have keypoints although less than before。In my opinion，mask area should not have keypoints。Is there any bug in sift code？

Steps to reproduce

1. the demo.txt is from asift.cpp, only add mask region and imwrite。
2. I compare two different mask region，
   
   the keypoints in zero mask area is less then non-zero area，mean the mask woks but not work correctly
   

Issue submission checklist

• I report the issue, it's not a question
• I checked the problem with documentation, FAQ, open issues,
  forum.opencv.org, Stack Overflow, etc and have not found solution
• I updated to latest OpenCV version and the issue is still there
• There is reproducer code and related data files: videos, images, onnx, etc