New command /wb remount <amount>, which determines the delay in server ticks before remounting a player to their vehicle after they're knocked back from the border. Requires new permission "worldborder.remount". The default value of 0 disables this feature, leaving the player dismounted beside their vehicle after knockback. This value should not be set to a value lower than 10 (the former default value) on a public server as that would lead to client glitches. Even the former default value of 10 has been reported to rarely glitch on some servers, thus remounting now defaulting to disabled.
Working around Minecraft and Bukkit client/server glitches when teleporting an entity which has a passenger or is a passenger is the whole reason for this "feature". The need was introduced a few Minecraft versions back when this well and truly broke; hopefully they will fix the underlying problems eventually.