Sounds like your butter/creme anglais mixture could have been too cold when you added the meringue (or possibly the meringue was too warm).
Your description sounds just like what can happen with mousseline when you add the butter: first it thins and curdles, then it comes together and emulsifies/thickenes once enough butter is added. But Silk Meringue doesn’t act like that, it is just whipped butter with stuff added, no curdling. And it isn’t intended to have a lot of whipping after adding the meringue, because that deflates the texture a bit and sort of defeats the reason for adding the meringue in the first place.
So that leaves me wondering if there could be some issue with scaling the recipe- when you tripled the recipe, did you end up with the correct quantities of custard, meringue and butter?
Or the other thought I had is that the larger batch may be more prone to temperature differences that contribute to curdling. A triple batch of custard will take a lot longer to cool, and a triple batch of butter will take a lot longer to warm, etc. Next time (I shudder to think of how many times you’ve made this…), consider bringing each of the three components- butter, custard and meringue, to between 70-75F before mixing them together.
Just to check, are you using whole milk for the custard?
If you find some scaling issue- like too much meringue or not enough butter- I would take a small portion of the existing thin bc and bring it back to 75, then treat it like mousseline, adding 75F butter by the tablespoon until it emulsifies. If that works, you’ll know what to do with the rest of the batch. Adding chocolate will accomplish much the same goal, adding more fat to achieve emulsification, so I would do one or the other, both may not be necessary.
Sorry this is a lot to check, just hoping we get this sorted out for you.