Can't use singleton design for Data Access Layer?

I have a singleton design for Data Access Layer (DAL) for accessing MySQL on local system. There are multiple threads using DAL to read and write data into underlying MySQL database. The target platform is RPi3 and build is done on a Yocto machine (building Boot2qt). Before upgrading to 5.12.1 we were running on 5.9 without any issues. After upgrading to 5.12.1 the program had hiccups and we received following messages in debug window:

QSqlDatabasePrivate::database: requested database does not belong to the calling thread.

By design of DAL, there will be only one instance of DAL that will result in single instance of QSqlDatabase object. We receive above mentioned error(s) while a worker thread is trying to access database through DAL.

Why worker thread is not able to use DAL on 5.12.1 while it was working on 5.9?