No it's not, we've been getting this for a while but it's properly forwarded to the log nodes now.
Update: We should probably get rid of that warning sometime though, but it won't give you any trouble.
Update again: Just wanted to let you know that in Mendix 5 this is no longer an issue.
What causes the following error: "WARNING: Unable to determine dialect of the StAX implementation" in 7.11?
2018-03-14T16:18:03.92+0000 [APP/PROC/WEB/0] mrt 14, 2018 4:18:03 PM org.apache.axiom.util.stax.dialect.StAXDialectDetector detectDialect
2018-03-14T16:18:03.92+0000 [APP/PROC/WEB/0] WARNING: Unable to determine dialect of the StAX implementation at jar:file:/home/vcap/app/runtimes/7.11.0/runtime/bundles/com.mendix.webservices-rt.jar!/