Why Do X2 Polyester Capacitors Lose Capacitance Over Time in Mains Power Circuits?

What do X2 (polyester?) caps degrade in uF over time? I can understand it for electrolytics but why these? I ask because I have had several instances of these roughly halving their uF over a few years. This is in devices that use one to feed a small amount of power to feed DC circuit like a door bell or mains timer. With reduced uF the device stops working as the DC voltage gets too low.

When using a capacitor to serve as a current-limiter to feed a low-power low-voltage DC circuit from a high-voltage AC source, the capacitor must have sufficiently-high voltage rating and also sufficiently-high AC-current rating. Many capacitors are not rated to pass significant AC current.

Notwithstanding my comments below I am happy to assist as a private member, especially to EPE members,X2 caps are designed to "self-heal". They have a very thin film which is metallised, and if this flashes over the metallisation vaporises and the cap just loses a bit of capacitance, but it does not fail short-circuit which would be disastrous in most X2 cap applications.  So capacitance loss is to be expected, especially if the cap is operated near its limits.  But a 50% loss sounds like a lot.

Attached photo shows one of two 0.47u 275VAC removed from two Byron wireless doorbells.
You would have thought that Byron, as a reputable brand, would have chosen a good
quality cap and derated it well. But no brand is evident to me on the cap.

 The bell starts to chime well but peters out like a “strangled
cat" as the DC voltage droops when current is required to drive the speaker.

 Similar issue with 2 or 3 Timeguard immersion timers https://uk.rs-online.com/web/p/products/82150...NQSp6VCut2uI3EBoCbmEQAvD_BwE&gclsrc=aw.ds
where the circuit would operate until current was demanded for the relay coil. Lasted 1-2 years before failing

Even though you don't have the manufacturer of the cap, I find it illustrative to look at a "main line" mfg like Kemet, and look at THEIR datasheet to see how much current it can take to get an idea on how much the part is stressed (stress ratio).  You will have to measure the current however to make that assesment.  For example, checkout Kemet's R71 line Metalized Polypropylene Film cap datasheet: (One I happen to be using on a current design)
https://www.digikey.com/products/en?keywords=R71VI41004030M

Just found another non working doorbell of a different make with an X2 cap about 30% low.