libtiff 4.7.0 introduced a regression which causes it to fail in
combination with ignore-tags, e.g. when used with ImageMagick's
"-define tiff:ignore-tags" parameter.

The problem has been reported upstream (for example in issues #532 and 
#706) and has been fixed in merge request #668. Since this also affects 
Debian Stable I would ask you to add the patch to Debian's libtiff6 
package.

Relevant upstream issues:

 - Issue: "Tiff image conversion with magick no longer works with
   tiff:ignore-tags"
https://gitlab.com/libtiff/libtiff/-/work_items/532

 - Issue: "TIFFMergeFieldInfo can no longer be used to ignore tags"
https://gitlab.com/libtiff/libtiff/-/work_items/706

 - Merge Request: "Updating TIFFMergeFieldInfo() with read_count=write_
   count=0 for FIELD_IGNORE."
https://gitlab.com/libtiff/libtiff/-/merge_requests/668