Got the same in Jenkins ver. 1.565.2
Daniel's comment is relevant, seems to always come with a negative spike of available executors.

– nevermind... didn't see the PR.

See https://github.com/jenkinsci/jenkins/pull/1480

Code changed in jenkins
User: Stephen Connolly
Path:
core/src/main/java/hudson/model/Label.java
core/src/main/java/hudson/model/LoadStatistics.java
core/src/main/java/hudson/model/OverallLoadStatistics.java
core/src/main/java/hudson/slaves/NodeProvisioner.java
core/src/main/java/jenkins/model/Jenkins.java
core/src/main/java/jenkins/model/UnlabeledLoadStatistics.java
core/src/main/resources/hudson/model/Messages.properties
core/src/test/java/hudson/model/LoadStatisticsTest.java
http://jenkins-ci.org/commit/jenkins/6b8af39b0317bad90851d4f8025fd71fadb9deaa
Log:
[FIXED JENKINS-21618] Ensure that the LoadStatistics return a self-consistent result.

The original method of computing load statistics would compute the total and idle counts independently
which could lead to counting errors while jobs started in between the different state counting operations.
This change switches to returning a `LoadStatisticsSnapshot` so that callers will get a single consistent
view of the counts which was valid for at least one point in time during the collection of the snapshot.

Code changed in jenkins
User: Stephen Connolly
Path:
changelog.html
http://jenkins-ci.org/commit/jenkins/46dc6850edb1d7ef52592794b15e69db7dfbed1a
Log:
Noting merges

• JENKINS-15355
• JENKINS-21618
• JENKINS-22941
• JENKINS-25938
• JENKINS-26391
• JENKINS-26900
• JENKINS-27476
• JENKINS-27563
• JENKINS-27564
• JENKINS-27565
• JENKINS-27566

• Fixing link text for JENKINS-6167

Integrated in jenkins_main_trunk #4044

Result = UNSTABLE