Hi Dan, do you think that some of the compost might seep out from under the bed edges when watering? It's just a thought. Maybe the raised beds need lining with a membrane which would keep the compost in place.
As raised beds aren't usually very deep why not use a combination of raised beds and containers or pots for the deeper rooting veg. Here's a couple of links for compost and spent mushroom compost.
UK Compost
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