Unexpected double WHERE clause in Servicestack OrmLite - servicestack

Question

We have an issue that occurs at every method call for limited periods of time. Then it works as expected. The issue is that the code produces double WHERE clauses.
We're using Servicestack 4.5.14
The method we have:
protected static void InsertOrUpdate<T>(
IDbConnection connection,
T item,
Expression<Func<T, bool>> singleItemPredicate,
Expression<Func<T, object>> updateOnlyFields = null)
{
var type = item.GetType();
var idProperty = type.GetProperty("Id");
if (idProperty == null)
{
throw new Exception("Cannot insert or update on a class with no ID property");
}
var currentId = (int)idProperty.GetValue(item);
if (currentId != 0)
{
throw new Exception("Cannot insert or update with non-zero ID");
}
var query = connection.From<T>().Where(singleItemPredicate).WithSqlFilter(WithUpdateLock);
T existingItem;
try
{
existingItem = connection.Select(query).SingleOrDefault();
Log.Verbose(connection.GetLastSql);
}
catch (SqlException)
{
Log.Verbose(connection.GetLastSql);
throw;
}
if (existingItem == null)
{
Insert(connection, item);
return;
}
var existingId = (int)idProperty.GetValue(existingItem);
idProperty.SetValue(item, existingId);
try
{
var affectedRowCount = connection.UpdateOnly(item, onlyFields: updateOnlyFields, where: singleItemPredicate);
Log.Verbose(connection.GetLastSql);
if (affectedRowCount != 1)
{
throw new SwToolsException("Update failed");
}
}
catch (SqlException)
{
Log.Verbose(connection.GetLastSql);
throw;
}
}
When it all works, an example output from the logs could be:
SELECT "Id", "Application", "Hostname", "LastContact", "Version", "ToolState", "ServerState"
FROM "ca"."ExecutionHost"
WITH (UPDLOCK) WHERE ("Hostname" = #0)
UPDATE "ca"."ExecutionHost" SET "LastContact"=#LastContact, "Version"=#Version, "ToolState"=#ToolState, "ServerState"=#ServerState WHERE ("Hostname" = #0)
When it fails, the output (same session, only seconds later) was:
SELECT "Id", "Application", "Hostname", "LastContact", "Version", "ToolState", "ServerState"
FROM "ca"."ExecutionHost"
WITH (UPDLOCK) WHERE ("Hostname" = #0)
UPDATE "ca"."ExecutionHost" SET "LastContact"=#LastContact, "Version"=#Version, "ToolState"=#ToolState, "ServerState"=#ServerState WHERE "LastContact"=#LastContact, "Version"=#Version, "ToolState"=#ToolState, "ServerState"=#ServerState WHERE ("Hostname" = #0)
Marked in bold is the addition to the SQL that makes the call to fail. It seems that it adds an additional WHERE clause with the content from the SET clause.
We've been debugging this for a while and don't really know if the issue is on "our" side or in Servicestack.
Any ideas on where to continue?