I would not be surprised if this trick was meant for early encapsulated foil capacitors. These were encapsulated in various semi-natural resins of not exactly k

I would not be surprised if this trick was meant for early encapsulated foil capacitors. These were encapsulated in various semi-natural resins of not exactly known or repeatable properties (usually the intention was that the resin is hygroscopic, but often it was also porous and prone to cracking with aging). Edit: then there is “RIFA plague”, which shows that you can get same issue even with better engineered encapsulation material.