XF 2.3 Indexing: valid items decreasing

[Salamanca]

Hi,

I switched from Invision to Xenforo a few weeks ago.
After the switch, the indexing for "Discussion Forum" items in Google Search Console sky-rocketed (in comparison to Invision) as seen in the image.
But it has started declining again, and I don't know the reason.
The Invalid Items have not increased though.

Any ideas why?
Thanks
Alex