Citrus peel seems fine in the outdoor heap but my worm bin clearly hates it, what is actually going on

I dumped the peel from a big batch of marmalade into a bin that had been running perfectly for a year. It went sour within days, mites exploded, and I had to strip the bin back to bedding and start again with about half the worms I had before. It took three months to get back to where it was.

Two different systems with two different limits. In a heap, peel is just a slow carbon rich item and chopping it makes a big difference to how fast it goes. In a worm bin the problem is acidity plus the oils in the skin, and worms will move away from a patch rather than eat through it. What you describe is a dose problem more than a poison problem.

I think citrus kills worms is one of the more overstated pieces of advice going. My bin gets peel every week alongside crushed eggshell to buffer the acidity and the population has doubled in a year. The keys are that it never exceeds roughly a tenth of what goes in, and that there is always plenty of bedding to balance it.

Heap yes, bin sparingly. Nothing about a peel is dangerous, it is just slow and sour in the wrong place.

Freezing peel before it goes anywhere is the trick I picked up for winter gluts. It bursts the cell structure so it collapses much faster once thawed, and it also means you can hold a week of peel in the freezer and dose the bin gradually instead of all at once.

Chop it, bury it, forget it. Whole peels in a heap are still recognisable next spring.