A shortened valgrind log follows for searchability, see the Debian bug for complete backtraces.
The matching packaged source code of packagekit-qt is available here.
==12351== Invalid read of size 8
==12351== at 0x27458BA0: PackageKit::Transaction::role() const (transaction.cpp:297)
==12351== by 0x273B6AAD: TransactionWatcher::watchTransaction(QDBusObjectPath const&, bool) (TransactionWatcher.cpp:104)
==12351== by 0x273B6B98: TransactionWatcher::transactionListChanged(QStringList const&) (TransactionWatcher.cpp:85)
==12351== by 0x5B59FCE: ??? (in /usr/lib/x86_64-linux-gnu/libQt5Core.so.5.15.7)
==12351== by 0x2744C094: PackageKit::Daemon::transactionListChanged(QStringList const&) (moc_daemon.cpp:419)
==12351== by 0x5B59FFB: ??? (in /usr/lib/x86_64-linux-gnu/libQt5Core.so.5.15.7)
==12351== by 0x27464B37: TransactionListChanged (daemonproxy.moc:331)
...
==12351== Address 0x1a6a2a10 is 16 bytes inside a block of size 24 free'd
==12351== at 0x484371B: operator delete(void*) (vg_replace_malloc.c:923)
==12351== by 0x5B4E53E: QObject::event(QEvent*) (in /usr/lib/x86_64-linux-gnu/libQt5Core.so.5.15.7)
==12351== by 0x49CAF5D: QApplicationPrivate::notify_helper(QObject*, QEvent*) (in /usr/lib/x86_64-linux-gnu/libQt5Widgets.so.5.15.7)
==12351== by 0x5B227C7: QCoreApplication::notifyInternal2(QObject*, QEvent*) (in /usr/lib/x86_64-linux-gnu/libQt5Core.so.5.15.7)
==12351== by 0x5B25760: QCoreApplicationPrivate::sendPostedEvents(QObject*, int, QThreadData*) (in /usr/lib/x86_64-linux-gnu/libQt5Core.so.5.15.7)
...
==12351== Block was alloc'd at
==12351== at 0x4840F2F: operator new(unsigned long) (vg_replace_malloc.c:422)
==12351== by 0x273B693B: TransactionWatcher::watchTransaction(QDBusObjectPath const&, bool) (TransactionWatcher.cpp:95)
==12351== by 0x273B6B98: TransactionWatcher::transactionListChanged(QStringList const&) (TransactionWatcher.cpp:85)
==12351== by 0x5B59FCE: ??? (in /usr/lib/x86_64-linux-gnu/libQt5Core.so.5.15.7)
==12351== by 0x2744C094: PackageKit::Daemon::transactionListChanged(QStringList const&) (moc_daemon.cpp:419)
==12351== by 0x5B59FFB: ??? (in /usr/lib/x86_64-linux-gnu/libQt5Core.so.5.15.7)
==12351== by 0x27464B37: TransactionListChanged (daemonproxy.moc:331)
==12351== by 0x27464B37: OrgFreedesktopPackageKitInterface::qt_static_metacall(QObject*, QMetaObject::Call, int, void**) (daemonproxy.moc:178)
...