mergeE is updating vertices in certain conditions

gremlin> g.addV('mytestlabel').property(id, 'testA').property('timestamp', 1).as('a').
......1>   addV('mytestlabel').property(id, 'testA2').property('timestamp', 1).as('a2').
......2>   addE('mytestlabel').from('a').to('a2').property('timestamp', 2).valueMap(true)
==>[id:2,label:mytestlabel,timestamp:2]
gremlin> g.V().mergeE([(T.label):'mytestlabel', (Direction.from): 'testA', (Direction.to):'testA2']).
......1> option(onMatch, sideEffect(property(single, 'timestamp', 4).property(single, 'edgelabel', 'exists')).
......2> constant([:])).valueMap(true)
==>[id:2,label:mytestlabel,timestamp:2]
==>[id:2,label:mytestlabel,timestamp:2]
gremlin> g.V().valueMap(true)
==>[id:testA2,label:mytestlabel,edgelabel:[exists],timestamp:[4]]
==>[id:testA,label:mytestlabel,edgelabel:[exists],timestamp:[4]]