With the "latching Shift" unticked in EuControl:Lock the shift key by selecting the shift key on two surfaces simultaneously, they DO lock as expected.
However they can NOT be unlocked without opening EuControl Settings and ticking than unticking the "Latching Shift".
EuControl 19.12.0.71
PT 2019.10
Mix firmware 1.5.6.0
Transport firm 1.1.9.0
Mac OS 10.14.6
iMac (Retina 5K, 27-inch, 2017)