My boys first day back at nursery was accidents all day, a couple of big ones and a couple where he started and stopped himself. He just didn’t have the confidence to ask to go quick enough. I sent him in with 10 changes of pants/trousers/socks so they could put in in clean clothes every time. We’d had zero accidents at home so it was a shock for him and me. However that was the only day it happened and he’s been fine since.
Personally I would tell the nursery that your child is now confused saying he wet his nappy and ask them to keep the nappies off him as this is part of the experience in potty training. Send him in with numerous bottoms as they then won’t have an excuse if he does it again in a short amount of time. It’s their job x
Have you taken him out in pants before or was nursery his first outing in pants? My daughter has been training a few weeks, I mostly know when she's likely to need a wee & put her on the potty, but when she goes off script & tells me she doesn't tell me until there's a dribble in her pants & has to be changed even though she did the wee on the potty (mostly). Nursery is a very busy, distracting environment & I'm not surprised he's having accidents there especially if he's only been trying a week & isn't 100% dry at home yet. Telling you he's done a wee in his nappy isn't necessarily a bad thing, it shows he's got awareness & is all part of learning.