Server validation potentially broken since 2.3.1

We started to migrate our project from Struts 2.2.3.1 to 2.3.1 (also tested with 2.3.1.1)
We currently use the server validation using XML validators with ActionClass-alias-validator.xml syntax.
Our interceptor stack is correctly configured having :
...
<interceptor-ref name="validation" />
<interceptor-ref name="workflow" />
...
Since this migration, our tests are resulting in database constraints violation because it seems that the validation is not called anymore.
Problem is solved when coming back to version 2.2.3.1
I have checked online documentation (http://struts.apache.org/2.3.1.1/docs/validation.html) but I can't see any change done from 2.2.3.1

The only similar issue I have found is https://issues.apache.org/jira/browse/WW-3736 but is on Client side, could be a potential clue.

Best regards,

Stéphane