A few years ago, Apple decided to completely rewrite its support for PDFs in PDFKit and Quartz2D. The new version broke a lot of existing code, and developers were forced to work around those bugs. Among the apps which had to do this was Apple’s own Preview. One obvious result is that Preview acquired its own bugs in handling PDFs, and hasn’t really recovered yet.
Preview isn’t, of course, a ‘serious’ PDF editor. But many use it to read PDFs and to add annotations to them, which aren’t particularly demanding requirements. Even now in Catalina, Preview version 11.0 remains a mess, and has some upsetting bugs.
Take its tools to Annotate a document. Open a copy of a regular PDF in Preview, and use the Highlight Text command in Annotations to highlight several sections of text. Save that document, open it again and chances are that Preview has forgotten the last of those highlighted sections. That is until you select the last highlighted text, when it suddenly remembers it again, as do all third-party apps. This appears to be because the final highlighting is tacked onto the end of the PDF code, where Preview seems to forget it.
Read more at EclecticLight.co