Keep them out of prolonged water, never put them in a dishwasher, and wipe on a little mineral oil once or twice a year. Both are organic materials that can dry out and crack, and both move slightly with humidity, which is normal on traditional knives rather than a defect.
Bone and stag are natural materials. They are stabilized and finished before use, which makes them far more predictable than raw material, but they are still organic and they still respond to humidity.
A handle that sits slightly proud of its bolsters in a dry winter and flush in summer is behaving exactly as expected.
Wipe after use. If it gets wet, dry it and let it finish at room temperature rather than near heat. Once or twice a year, a light wipe of food-safe mineral oil keeps it from drying out.
Never soak, and never use a dishwasher. Both are how cracked stag handles happen.
Extremes are the danger: a hot attic, a car in summer, or a very dry heated room over a whole winter. Stable humidity is what keeps a hundred-year-old bone handle intact.