SessionSettings gets cleared intermittently

We have been using DynamicAcceptorSessionProvider in our project for quite sometime now, together with ThreadedSocketAcceptor.
After a recent upgrade to Quickfix 2.1.0 (from 1.5.3), we saw some errors that I believe is caused by Quickfix being unable to find the session settings.
The stack trace looks like below:

---

2018-11-19 00:01:13,236|| ERROR |||| org.quickfixj.QFJException: quickfix.ConfigError: Missing ConnectionType | NioProcessor-23 | q.m.a.AcceptorIoHandler
org.quickfixj.QFJException: quickfix.ConfigError: Missing ConnectionType
at quickfix.mina.acceptor.DynamicAcceptorSessionProvider.getSession(DynamicAcceptorSessionProvider.java:153) ~[quickfixj-core-2.1.0.jar:2.1.0]
at quickfix.mina.acceptor.AcceptorIoHandler.findQFSession(AcceptorIoHandler.java:118) ~[quickfixj-core-2.1.0.jar:2.1.0]
at quickfix.mina.AbstractIoHandler.messageReceived(AbstractIoHandler.java:129) ~[quickfixj-core-2.1.0.jar:2.1.0]
at org.apache.mina.core.filterchain.DefaultIoFilterChain$TailFilter.messageReceived(DefaultIoFilterChain.java:997) ~[mina-core-2.0.19.jar:?]
at org.apache.mina.core.filterchain.DefaultIoFilterChain.callNextMessageReceived(DefaultIoFilterChain.java:641) [mina-core-2.0.19.jar:?]
at org.apache.mina.core.filterchain.DefaultIoFilterChain.access$1300(DefaultIoFilterChain.java:48) [mina-core-2.0.19.jar:?]
at org.apache.mina.core.filterchain.DefaultIoFilterChain$EntryImpl$1.messageReceived(DefaultIoFilterChain.java:1114) [mina-core-2.0.19.jar:?]
at org.apache.mina.filter.codec.ProtocolCodecFilter$ProtocolDecoderOutputImpl.flush(ProtocolCodecFilter.java:437) [mina-core-2.0.19.jar:?]
at org.apache.mina.filter.codec.ProtocolCodecFilter.messageReceived(ProtocolCodecFilter.java:256) [mina-core-2.0.19.jar:?]
at org.apache.mina.core.filterchain.DefaultIoFilterChain.callNextMessageReceived(DefaultIoFilterChain.java:641) [mina-core-2.0.19.jar:?]
at org.apache.mina.core.filterchain.DefaultIoFilterChain.access$1300(DefaultIoFilterChain.java:48) [mina-core-2.0.19.jar:?]
at org.apache.mina.core.filterchain.DefaultIoFilterChain$EntryImpl$1.messageReceived(DefaultIoFilterChain.java:1114) [mina-core-2.0.19.jar:?]
at org.apache.mina.core.filterchain.IoFilterAdapter.messageReceived(IoFilterAdapter.java:121) [mina-core-2.0.19.jar:?]
at org.apache.mina.core.filterchain.DefaultIoFilterChain.callNextMessageReceived(DefaultIoFilterChain.java:641) [mina-core-2.0.19.jar:?]
at org.apache.mina.core.filterchain.DefaultIoFilterChain.fireMessageReceived(DefaultIoFilterChain.java:634) [mina-core-2.0.19.jar:?]
at org.apache.mina.core.polling.AbstractPollingIoProcessor.read(AbstractPollingIoProcessor.java:539) [mina-core-2.0.19.jar:?]
at org.apache.mina.core.polling.AbstractPollingIoProcessor.access$1200(AbstractPollingIoProcessor.java:68) [mina-core-2.0.19.jar:?]
at org.apache.mina.core.polling.AbstractPollingIoProcessor$Processor.process(AbstractPollingIoProcessor.java:1242) [mina-core-2.0.19.jar:?]
at org.apache.mina.core.polling.AbstractPollingIoProcessor$Processor.process(AbstractPollingIoProcessor.java:1231) [mina-core-2.0.19.jar:?]
at org.apache.mina.core.polling.AbstractPollingIoProcessor$Processor.run(AbstractPollingIoProcessor.java:683) [mina-core-2.0.19.jar:?]
at org.apache.mina.util.NamePreservingRunnable.run(NamePreservingRunnable.java:64) [mina-core-2.0.19.jar:?]
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149) [?:1.8.0-zing_18.06.0.0]
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624) [?:1.8.0-zing_18.06.0.0]
at java.lang.Thread.run(Thread.java:748) [?:1.8.0-zing_18.06.0.0]
Caused by: quickfix.ConfigError: Missing ConnectionType
at quickfix.DefaultSessionFactory.create(DefaultSessionFactory.java:102) ~[quickfixj-core-2.1.0.jar:2.1.0]

I find this very strange, as we definitely have the session configuration, and the session configuration specifies the 'ConnectionType' (which is an 'acceptor').