This happens whenever you fight on a hill or something, or you have a little hill in between. Springalds would then attack a mangonel from one tile away. You really need to babysit the siege to check if they don’t move in too close range while they should already attack. Because of this, you cannot use your siege which is very annoying. People call it siege kissing.
Thank you for reporting @DavidoNL4537! We have some siege work going on, so hopefully we can take a look at this as well. Your report is much appreciated!