I've namechanged for this.
Since my 2 boys spent a week at my parents last summer, the eldest has seemed unsettled when they visit or we visit them and for the week or so before we go / they arrive. They had stayed there twice before with no problems but when we picked them up last time, they were like zombies, really exhausted. They were 6 and 3. On questioning Ds1 said it was just too long and he would be fine to go just for a couple of days, but when we came to take them at Christmas the behaviour was really bad until we said you don't have to stay, we can just visit and come back with us. Even then his favourite cuddly toy was much in evidence when we were there.
Next week we will be stopping at their house on our way to our holiday. They won't even be there but Ds1 seems anxious nevertheless.
I will say we put this down to them bickering amongst themselves, not keeping to the routine, doing too much and overtiring them. But surely not a year later, so now we are worried it may be something more sinister. Does anyone have any thoughts on this.