As you have found, earlier versions of PlantUML were using standard arrowheads. At some point, we found that the drawing with the new arrowhead was nicer (albeit non standard).
In current version, you cannot change this behaviour (except in rebuilding the .jar).
We can add a skinparam to allow user to switch between arrowheads:
For standard UML drawing that you are expecting: skinparam sequenceArrowHead standard
For current drawing : skinparam sequenceArrowHead nice
The default behaviour would be nice
Would it be ok for you ?