It seems that Extremtextil get in different lots but list them under the same description. About 5 years ago I got some 70den/170gsm tpu nylon to make a sleeping mat, and it heat welded fine. A couple of months ago I got another lot, listed with the same desription, but it just melted when it should have welded. I assume this is the difference between nylon 6 and 6.6 – they melt at radically different temperatures.
I think the onus needs to put on them to either send the right stuff or say that they haven’t been given those details to pass on. They will do half metre lenghts for testing, but shipping is expensive to the UK for small amounts.