I've seen it happen a few times. Seems some of these roofs age worse than others. Good job on the restoration! Be sure to get some UV protection of some kind on there or it may cloud up faster than you'd like.
The 451 front windscreen and panoramic roof was put on in the factory with a robot arm applying the sealant and then placing in position without human supervision, so some unfortunately get put on with less sealant than others and not spotted.
The 451 front windscreen and panoramic roof do not suffer much with this problem of poorly applied sealant, though more commonly UV light from the sun does cause micro-crazing of the roof, and it sparkles a lot when seen from inside looking up through the clear roof. Mine suffered that, but my Wife loved the effect
Note: The 453 does suffer badly from the front windscreen having less sealant put on by mistake, due to a different robot arm design in the 453 factory production, and we see it all the time now on 453 forums with wind getting in through the windscreen perimeter. Windscreen has to be removed and re-sealed, cost here in UK is £100 by windscreen fitting companies.