The internal link between master and detail data records no longer works. 
Forms that worked correctly under version 6.0 show the error message 
"The data contents could not be loaded. You have an error in your SQL syntax .... The SQL command that caused this error is: 
SELECT * FROM "myDatabase". "myMasterTable" WHERE ("myDatabase". "myMasterTable". "ID" = :link_from_myDetailID) and then only the data from the highest master table.

To confirm this, you can use the form wizard to create a new form with the simplest possible master-detail relationship (1 linked field), this will fail in the same way.

This renders proven applications built on Base currently unusable!

Context: MariaDB as backend, LibreOffice and MariaDB/MySQL-Connector installed via DEB from Ubuntu PPA, [Version: 6.2.2.2, Build ID: 1:6.2.2-0ubuntu0.18.04.1~lo1, CPU ,threads: 8; BS: Linux 4.15; UI render: default; VCL: gtk3;  locale: de-DE (de_DE.UTF-8); UI language: de-DE, Calc: threaded]