If no network configuration is specified in the query, then the first active configuration is used.
Active configurations are listed using QNetworkConfigurationManager::allConfigurations, but this includes configurations that were activating by another process.
So if an application is trying to use GPRS, its traffic may be incorrectly routed towards a WLAN proxy because another application has started the WLAN.

This should not affect QNetworkAccessManager traffic in 4.8 and 4.7.4, because the proxy query includes the network configuration.

Most severe impact is if the socket is bound to a specific bearer, but a proxy from a different bearer is selected.
This is because the socket connection will timeout as the IP address is unreachable (or refers to a different host for private addresses)
As standard practice is to drop packets rather than return unreachable responses, the connect is likely to timeout rather than fail fast.