Sorry for the late reply.
I’m feeling rather stupid now, but the problem for me has “magically gone away”… 😐 I haven’t touched the site in question for a day and returning to it today, the problem I was having with with save_post not being triggered in some themes but in others is gone.
So it must have been some random, temporary issue with my environment (I’m flabbergasted, can’t for the life of me figure out what could have caused that rather odd and not the least inconsistent behaviour).
Sometimes it seems “ignore it and it will go away” does work as a debugging tactic. -_- Sorry for all the fuss on my part!
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