QuickFIX/N FIX data dictionary validation

Question

I have a FIX protocol solution that is built on QuickFIXn.Core v1.11.0, which generally works fine, establishing a session and exchanging admin messages, using FIX.4.4 version. But when it comes to App messages, my partner is using custom message structure, which causes an error that resembles failing data dictionary validation.

When I manually validate the message against the data dictionary and use this code, everything looks good:

var path = @"path_to_the_xml_dictionary";
DataDictionary dt = new(path);
var message = "custom_message";
Message m = new Message(message, dt, true);
dt.Validate(m, "FIX.4.4", "8");
m.FromString(message, true, dt, dt);

What is odd is that after this error FIX client immediately sends a logout request with 58=Incorrect BeginString (FIX.4.4) as a reason, indicating there is something wrong with the data dictionary, but nothing more than that.

Full error stack trace, from the logs:

QuickFix.UnsupportedVersion: Incorrect BeginString (FIX.4.4) at QuickFix.DefaultMessageFactory.Create(String beginString, String msgType, Int32 groupCounterTag) at QuickFix.Message.SetGroup(StringField grpNoFld, String msgstr, Int32 pos, FieldMap fieldMap, IGroupSpec groupDD, DataDictionary sessionDataDictionary, DataDictionary appDD, IMessageFactory msgFactory) at QuickFix.Message.FromString(String msgstr, Boolean validate, DataDictionary sessionDD, DataDictionary appDD, IMessageFactory msgFactory, Boolean ignoreBody) at QuickFix.Message.FromString(String msgstr, Boolean validate, DataDictionary sessionDD, DataDictionary appDD, IMessageFactory msgFactory) at QuickFix.MessageBuilder.Build() at QuickFix.Session.Next(MessageBuilder msgBuilder)