In NH the voided villagers stay for a loooong time, they can no longer be stopped like back in NL by visiting another town that has max amount of villagers.
My tip is just to NOT visit other islands,
whether local or online, when you have an empty plot. If you're planning to visit someone else to adopt a villager in boxes, then don't go anywhere else before you do that.
If you plan to villager hunt on the mystery islands, do not go elsewhere. That's it
This is when paranoia pays off lol.
Think of yourself as social distancing when you have an empty plot

Otherwise you're fine to visit or have visitors when you don't have an empty plot (I do a lot of trading and hosting and never had a random mover from friends' islands and planning to keep it that way!)