Set up a job to monitor all branches ('**' in 'Branches to build') but restrict it to one folder ('Project1/.*' in 'Included Regions').

As long as you make submissions to one branch, the included regions works fine.
However, when you make a change on a different branch, the first job that triggers will ignore included regions and pick up changes from any location. After that the next jobs on the same branch will follow the restrictions correctly until a change on another branch is made.

Here's part of my config.xml:

<scm class="hudson.plugins.git.GitSCM" plugin="git@2.3.5">
<configVersion>2</configVersion>
<userRemoteConfigs>
<hudson.plugins.git.UserRemoteConfig>
<url>git@github.com:SomeAccount/1.git</url>
<credentialsId>ff33d8db-ec3a-48d2-83cd-6d5cbd8f51a4</credentialsId>
</hudson.plugins.git.UserRemoteConfig>
</userRemoteConfigs>
<branches>
<hudson.plugins.git.BranchSpec>
<name>**</name>
</hudson.plugins.git.BranchSpec>
</branches>
<doGenerateSubmoduleConfigurations>false</doGenerateSubmoduleConfigurations>
<submoduleCfg class="list"/>
<extensions>
<hudson.plugins.git.extensions.impl.PathRestriction>
<includedRegions>Project1/.*</includedRegions>
<excludedRegions></excludedRegions>
</hudson.plugins.git.extensions.impl.PathRestriction>
</extensions>
</scm>

Here's a part of the 'GitHub Hook log':
Seen 51 remote branches
> /usr/bin/git log --full-history --no-abbrev --format=raw -M -m --raw [edited change A] ..[edited change B] # timeout=10
Ignored commit [edited change B]: Found only excluded paths:
Done. Took 1.2 sec
Changes found