I just upgraded my 3206 to 5.12.6, and Ext. trigger seems to work as before.
However, you could try to change the triggering settings via the Settings->Trigger menu. In 5.12.3 I did observe some strange changes in the triggering levels, when switching trigger settings from the toolbar. The toolbar would display e.g. 200 mV, while in the menu it was 2000 mV, or 0.2 mV.
For some reason, under the Settings->Trigger menu, the level window is greyed out when on Ext. Trigger, so you may need to change it to A or B before, and then back to Ext.
In order to put the oscilloscope to full use, it would be really nice to know exactly how it is expected to behave. Unfortunately, this would take the circuit diagram and the essential parts of the software/firmware algorithms.
I wonder, what would be the most detailed specifications that Picotech would be able to release? The idea is, that the oscilloscope is supposed to be the known device in the chain, and not a DUT.
Best regards, and thanks.