Trouble Powering Microcontroller from Doorbell Transformer
A doorbell transformer does not produce 21VAC. Or any other voltage. It has an air gap and/or bypass that makes it "soft".

It supplies a mostly constant current, not a constant voltage. That's by purpose so it doesn't make a difference how long and thin and rusty the wires, and wire nuts from the multiple doorbell knobs to the doorbell are. Also, how rusty the most times poorly made doorbell knob contacts are. The bell has always the same volume (selectable by the "voltage" at the transformer), regardless which knob is pressed.

If you want to supply something else in addition ot the doorbell, you better put it in series with it, not parallel. If your device has high impedance (almost any electronics, e.g. your DC converter), you need a local impedance transformer.