The latest update from the SPT website is that the trains were delivered getting on for 18 months ago now.
Testing period by manufacturer begins.
www.spt.co.uk
I haven't found any other information elsewhere.
What's happening with these units?
I suppose the key thing here is that the units are being tested onsite, as the unique track guage and other things are somewhat awkward.
Since the new units will also all be part of the move towards full unattended operation, and the pandemic having delayed just about everything everywhere, the timelines may now be changed.
It's also entirely possible that there may be a desire between Stadler and SPT to have the entire fleet delivered and comissioned ready for a complete switch-over from the old to the new stock to minimize potential interaction issues and such, which in normal fleet replacement projects take up substantial amounts of testing time.
All in all, 18 months for the full testing of a new design of vehicle isn't out of line with what is seen elsewhere, just the testing tends to be done in multiple locations.