Hi there - sorry for the inconvenience with that false positive behavior.
Werk 11543 fixes a “false negative” behavior by handling None and "None". Unfortunately it also introduces the false assumption that no active interface should be treated as CRIT.
This will be fixed in p20 - in the meanwhile you can workaround this behavior by copying checks/bonding.include to ~/local/share/check_mk/checks/ (like suggested by Andreas, but taking the p19 version) and changing yield 2, "No active interface" in line into yield 0, "No active interface" in line 67.
hth, Frans