When a rider token is a smaller size than the mount token and the 'Rider movement' setting is 'Move ridden Token (if owned)', selecting and moving using the rider token results in the mount token's space overlapping between grid squares. This can also happen when using the setting for 'Adjust rider size' when the rider and mount are the same size.
Attached screen shot of an example. Foundry v11.308, dnd5e v2.3.1, Rideable v2.11.0
When a rider token is a smaller size than the mount token and the 'Rider movement' setting is 'Move ridden Token (if owned)', selecting and moving using the rider token results in the mount token's space overlapping between grid squares. This can also happen when using the setting for 'Adjust rider size' when the rider and mount are the same size. Attached screen shot of an example. Foundry v11.308, dnd5e v2.3.1, Rideable v2.11.0