Hi, I'm new at gardening and have run into the same problem with pea plants 2 years in a row. When my plants got about 1.5 feet high I would soon find the stem snipped clean at the base just above the ground. No other damage is visible, nothing else nibbled on just the stem cut.
Last year I checked for grubs but didn't find any. This year I put a netting around the plants in case large birds were the culprit and also put in a sonic rodent device but it still happened. Why would something do that and leave the rest of the plant untouched? Any ideas?
Perhaps next year I will put some sort of protective guard around the base of stem.