Cancel does not stop containers

I’m using agents to test some CI workloads using drone 1.1 and I have a problem with the ‘cancel’ button. After clicking ‘cancel’, I see this output from the agent log:

DEBU[0000] successfully pinged the docker daemon        
DEBU[0000] runner: polling queue                         arch=amd64 machine=c106fa85e52c os=linux
DEBU[0000] runner: get stage details from server         arch=amd64 machine=c106fa85e52c os=linux stage-id=71
DEBU[0000] runner: watch for cancel signal               arch=amd64 machine=c106fa85e52c os=linux
INFO[0001] runner: start execution                       arch=amd64 build=71 machine=c106fa85e52c os=linux pipeline=default repo=stevecrozz/vdr-app stage=1 stage-id=71
DEBU[0708] runner: received cancel signal                arch=amd64 machine=c106fa85e52c os=linux

After clicking cancel, the drone UI updates and from the user’s perspective, everything looks as it should, but the docker containers that were created to run the build do not exit. What steps can I take to debug this?