I've been experimenting with panettone and I've come across the issue time and time again of losing dough consistency while mixing the secondo impasto (i.e. the final panettone dough). What makes this super bizarre to me is how I can go from fully-developed gluten to puddle of mud in only a minute or so after adding some fats/sugar. The new dough consistency is similar to cake batter, or pudding, or thick mayonnaise. You can stick your finger into this gloop, pull it out, and it literally forms "stiff peaks" (a reference to whipping egg whites). It's kind of glossy and can definitely be described as similar to choux dough. I guess it could be also compared to a very very over-fermented high hydration sourdough (i.e. a dough left to ferment for days or weeks). The viscosity is high -- I can "pour" the dough from a Teflon coated container as a single big thick blob (without it sticking to the bowl). (It's unfortunate that I don't have pictures to give a visual.) Thus far, I've determined that the issue does not stem from  Using a flour that is too "weak" (though I'm unsure if using an even stronger flour would decrease the likelihood of the dough losing consistency) Adding too much water (I've tested adding a tad bit extra water, but not a whole lot) Over fermentation  I'm assuming overfermentation isn't the issue because over-fermented dough can be identified before the second mixing is initiated. (But perhaps the byproducts or acidity produced during the fermentation process can aggravate the issue?)    I've found references to this issue online,  https://www.thefreshloaf.com/comment/26611#comment-26611 https://www.thefreshloaf.com/node/5232/panettone http://hilda.hhandg.com/?p=1483 https://www.thefreshloaf.com/node/69835/panettone-trouble  And they seem to point to sugar being the culprit in the loss of dough consistency. The comment mentions how mixing for another hour turned the "milkshake" to "finally...where it [gluten] needed to be". But I've experimented with a very long mixing time to see where things would go. It doesn't take too long for the dough to overheat, so the first thing I'll notice is that the dough is leaking oil. After a couple or a few hours, the dough seems to solidify to form a somewhat grainy paste (like stiff/thick icing), which can be shaped and isn't sticky -- however, it has no elasticity. Currently, I suspect too much fat and sugar/erythritol*** as being the main contenders for causing this issue. I'm aware that fats prevent long gluten chains from forming, and that regular sugar can compete for water which hinders gluten formation. (Well, the thing is, gluten was already fully developed...and then, well, all the bonds seemed to break down and refused to reform.) Furthermore, erythritol is not hygroscopic and is less than 1/5 the solubility of plain sucrose, but I'm not sure about how that would change things. Perhaps undissolved sugar actually cuts gluten (just speculating)? Does adding sugar slowly actually have any meaning/effect? Could overmixing be contributing to the issue? Is it even possible to overmix panettone/brioche dough (assuming you don't overheat it)? I haven't tested this but I was wondering if too much mixing could result in an irreversible puddle, as referenced in this post I'll also note that I add diastatic barley malt (~0.6% flour weight) in the final dough, but I doubt the proteolytic activity from diastatic barley malt could be fast enough to cause the issue (correct me if I'm wrong). More testing definitely needs to be done on my end, but I was just wondering if there are any experts out there that know the chemistry behind this fiasco and how to prevent it. ***: I'm using a erythritol+monkfruit blend in place of plain sugar; in total it's about 7% of the final dough weight (before adding dried fruit), or about 20% of the flour weight. Update: After some further research, it looks like mixing definitely plays an important role. From what I've read mixing (improperly or over) can result in the trouble I'm facing. 
In my experience, the secondo impasto can be a delicate balance.  In the formula I use, most of the sugar, fat, and eggs are added in the first dough, with a smaller proportion added in the second mix.  At that point, I find the temperature of the butter to be important.  Soft enough to get worked into the dough, but not too soft.  Too soft, and the whole thing can come undone, as you describe.  Also, these final ingredients need to be added gradually.  I don't have experience with the sugar substitute you are using, so I don't know if that is the culprit.  But, be sure you are paying attention to proceeding gradually, with butter at the right temperature. 