If the last change on current branch is lower than HEAD CL - 'Head Change Query Limit' then any change to any branch under the parent path will trigger a build in current branch.

Reproduction steps:

(1) Create two Helix branches with Jenkinsfiles:

//depot/multibranch/main/Jenkinsfile.

//depot/multibranch/main/f1

//depot/multibranch/dev1/Jenkinsfile.

//depot/multibranch/dev1/f1

(2) Create a Jenkins multibranch project with "include Branches" set to:

//depot/multibranch/...

(3) Scan the multibranch pipeline

(4) Two jobs will build. One for each branch.

(5) Add 2000 new revisions of dev/f1.  Below is a helper script to create N revisions of file X:

if [ ! -n "$1" ]
then
  echo "Build X  revisions of the given filename."
  echo "Usage: buildfile.sh <Num> <Filename> "
  exit -1
fi  MYFILE=$2
MYNUM=$1
echo Created `date` ... > $MYFILE
p4 add $MYFILE
p4 change -o |
       sed "s/<enter description here>/Automated Submit/" |
       p4 submit -i
for ((i=1;i<$MYNUM;i+=1)); do 
  echo $i; 
  p4 edit $MYFILE
  echo This is some automatically inserted text - Line $i >> $MYFILE
  p4 change -o |
       sed "s/<enter description here>/Automated Submit/" |
       p4 submit -i
done

(6) Scan multibranch pipeline.

Result - dev1 and main builds

Expected Result - dev1 builds.

(7) Make another edit to dev/f1.

Result - dev1 and main builds

Expected Result - dev1 builds.

Workaround:

Increase "Head change query limit" to 10000 (ensures the last changelist on main is picked up by p4 changes -m1" then allow the main build to run once to update the SyncID correctly.