Yes, I also had an intermittent earth-leakage trip on a Heating system, that I'd considered running off an isolation transformer so it could have it's own RCD and not take out the main RCD. Position of consumer unit makes it awkward to fit a separate RCBO circuit for it.
11.82V is still within the Output voltage spec. of a typical 7812: 11.5-12.5V (12V +/-4.2%), over the full-temperature range: eg:
https://www.hobbytronics.co.uk/datasheets/L7812.pdf.
But you could ensure this is off-load voltage / measure direct at regulator, so that wiring-drops aren't a factor.
Also maybe check that the input to the 7812 is always > 15V, and there isn't a lot of ripple due to a failing smoothing capacitor between the rectifier diodes and it's input that could e causing it to dropout.
However, I have often measured the 12V rail as being closer to 11V on some PC PSU's, that only regulate the +5V rail and the +12V is just derived from a ratio-ed tapping on transformer without any post-regulation.
Some HDD were a bit fussy if this was a bit low, not starting it very well.