Upgrade to .NET6 changes the edi output

Hello,
I have a project that produces EDI message for a few year, running well. (Core2.1)
Now we have upgraded our project to (.NET6)
Both projects use indice-co version 1.9.18.

But the output is quite different ...
in our previous project using version Core2.1 the program produces the following edi output :

UNB+UNOC:3+TRADER.NL100009999.00+NTA.NL+211028:1433+21102814330905++++0++0
UNH+21102814330905+CUSDEC:D:96B:UN:NL015B

while in our new version .NET6 we have as output

UNA:+.?
UNB+UNOC:3+TRADER.NL100009999.00+NTA.NL+211027:1254+21102712540856++++0++0
UNH+21102712540856+CUSDEC:D:96B:UN:NL015B
UNH+

As you can notice , the program adds suddenly some extra segments ... UNA + segment and twice a UNH record instead of 1 ....

Do you have any idea where to look for
Thank y so much !

Start by reproducing the reported difference between Core 2.1 and .NET 6 using indice-co version 1.9.18, then compare the serializer configuration and generated EDI output. Investigate why UNA is added and why an extra UNH appears; done means identifying the compatibility cause and restoring the expected single-UNH output or documenting the required configuration.

Written by the indexing model from the issue text.