I'd probably suspect the sender itself 9 times out of 10 (well 9 times out of 10 'after' you check the wire hasn't popped off, which you have already done!)

It's also possible on some engines (and I have to be honest and say I've not seen it on a datto, but it might theoretically be possible - is you can get an air pocket around the sender so it reads artificially low (or as if it wasn't reading at all!)

I'd go the sender for sure.