We have noticed that fielded searches, when entered manually on the discovery pages don't work in DSpace 5.3. By "don't work", I mean, they are acting more like fulltext queries.
For example, if I go to demo dspace and search for title:test, it will return items whose titles do not contain the word test, but whose other fields do.
If this line is commented out, fielded searches begin working again. However, this might also re-introduce the bugs it was intended to fix. I'm not sure at this point. Perhaps a "smarter" escape is needed here, which ignores colons?