DisplayDocTitle true for accessible PDFs even when PDF/UA is not enabled

I noticed that the document title check fails in Acrobat Pro accessibility checker when accessibility element is true in fop.xconf but pdf-ua-mode element is omitted. When, in addition to setting accessibility to true, I include pdf-ua-mode in fop.xconf with the value PDF/UA-1, the Acrobat Pro check passes.

Based on comparing the decoded original PDF and a decoded PDF fixed by Acrobat Pro, the difference in failing and passing lies in the inclusion of DisplayDocTitle (with value true). WCAG 2.1 technique page PDF18 supports this conclusion.

Is setting DisplayDocTitle to true tied to PDF/UA mode instead of accessibility in general on purpose? If not, could DisplayDocTitle be true whenever accessibility is enabled? My thinking is that some might not target PDF/UA compliance but would like to minimize the failures in accessibility checks nevertheless.

Source files that are relevant in this to my understanding:
fop-core/src/main/java/org/apache/fop/pdf/PDFRoot.java
fop-core/src/test/java/org/apache/fop/pdf/PDFUATestCase.java

Relevant Adobe Acrobat Pro support page (see heading "TITLE"):
https://helpx.adobe.com/acrobat/using/create-verify-pdf-accessibility.html#check_accessibility_of_PDFs