Panettone top crust separation

Yesterday, I finished my second attempt at making panettone. I modified Roy Shvartzapel's formula in Sourdough Panettone and Viennoserie by Thomas Teffri-Chambelland to use semi-sweet chocolate chips instead of dried fruit (because my wife isn't a huge raisin fan), and I scored and buttered the top instead of glazing.

My first attempt last week was a disaster for two reasons: 1) the recipe never says to add the salt and I completely missed it, and 2) I tried to make my own parchment molds in an 8" cake pan and I was unable to remove and hang them without them getting flattened. This time was much better, and I'm actually really surprised at the texture and appearance of the crumb.

Rather than make a lievito madre, I refreshed my existing stiff starter (which, at 57% hydration, already makes naturally leavened bread that I hesitate to call sourdough because it really isn't that sour) three times at 3.5-4 hour intervals at 45% hydration. It tripled in 4 hours each time, and the first dough tripled in the 10-12 hours that the recipe said it should, so I'm quite satisfied with its performance. The final proof took almost twice as long as the recipe said (7.5 hours vs 4), but I don't have a good proofing box so I just had it in the oven with the light on, which is about 78°F.

The one defect that I'm looking for tips on is the separation of the top crust. I did notice that as I struggled to quickly skewer and hang the panettone it was starting to deflate within a minute of coming out of the oven, so I don't know if that might have caused it. Or did it have something to do with shaping or scoring? I wasn't sure if the final proof was supposed to be covered or uncovered, so I had a plastic bag over it to keep it from drying out.