It is possible that the settings for inviting animals are set to "Daily Change", or it is due to the "Auto" function.
Please check each of the settings.
"Daily Change" settingIf you turn it on, the animals will be replaced automatically at 6:00 (UTC).
*15:00 (JST) in Japan time.
Please tap the guest screen (white cat mark) on the left side of the screen and check if the "Daily Change" setting in the upper left is turned on.
"Auto" functionWhen you tap "Auto", the animals you have invited will be replaced immediately.
Tap the guest screen (white cat mark) on the left side of the screen, and you can set it from "Auto" at the bottom right.
In addition, if you do not want to automatically replace a specific animal, you can lock it.
After tapping "Lock" on the guest screen, if you tap an animal that you do not want to be automatically replaced, it will be locked and the animal will not be automatically replaced.