> Today they operated Line 12 for the first time with the bi-directional CAF Urbos 100

What is the status on Gent PCC trams that operated on Line 12 before? Was Saturday June 8th officially the last day?

No, today, Monday the 10th of June, they are back in service.
Yesterday, Sunday the 9th of June, the metro tunnel between station Astrid and station Sport was closed due to track works. So lines 2-3-5 and 6 could not operate this section.
And since line 12 runs more or less parallel, they operated this route with the bi-directional CAF trams because they needed more capacity. Due to the "Sinksenfoor" (an outdoor fair that runs for 6 weeks at "Spoor Oost"(entrance is at Schijnpoort).
Later this year they will build a new end station at Halenstraat. The end station Schijnpoort is to small to operate the CAF trams in normal condition. This was an emergency, so there were a lot of people from "De Lijn" at Schijnpoort to make sure every thing went well.

Thanks for a full explanation! All is clear now.