Was it working before p19? To test i have only a p18 here and there is no problem at the moment.
I think i found the reason. The mentioned werk destroyed the Linux bonding as it has it’s own parse function and don’t used the generic parse function for bonding interfaces.
The Problem is the following commit.
To make a quick fix you can take the file “checks/bonding.include” from a p18 version and copy this file to the folder “~/local/share/check_mk/checks/” now it should work again. If this is fixed in p20 you can then remove the manual copied file.