Schengen Visa Germany, sponsorship denied due to lack of “commitment form”

My Thai partner recently attended an appointment for a Schengen visa at the German embassy in Bangkok.

Upon arriving and before submitting the application we were informed the application would be denied because I lack a commitment of sponsorship form that I apparently would have to obtain from the German embassy in the UK, a form that before the appointment I have found no reference to on shengen Visa advice websites.

In the application I (as a UK citizen) provided a sponsorship letter detailing my commitment to pay the costs and explaining the source of my funds. For supporting documents I included up to date bank statements (coving more than 6 months from today’s date), proof of property ownership in the UK, a copy of my passport, and a fully booked (and paid for) itinerary for our trip to Europe.

We applied for and have had accepted a UK tourist visa having supplied much the same set documents.

I have evidence of considerably more than the recommended sufficient (£55x planned number of days) funds available going back long before the time period on my supplied bank statements.

We would be happy to adjust our itinerary to apply to a different Schengen country for the visa, ideally one we could apply to from within Thailand, countries which we are already planning to visit are: Germany, Czech Republic, Austria, Hungary, Romania.

If anybody has had experience with this situation could they suggest an alternative recourse for a Schengen visa application that would not require me to fly from Thailand to the the UK just to obtain a form?