You must have been deemed suitable for temporary promotion ...... you should be competent within current role and have taken the necessary assessment for the next role through such as an ADC or similar. It does provide the criteria in the grey book.
Competence is achieved by demonstrating to an assessor on several occassions over a period of time that you are consistently achieving the benchmark standards and providing other evidence, such as witness testimonies to substantiate the fact.
When you are in a development role, you are developing ..... learning the skills and aquiring the underpinning knowledge to demonstrate competence and obtain experience.
Your employers must be aware of the responsibility they are placing on you by giving you temporary promotion, so I think you are reading too much into it.
Legally speaking, your employers are vicariously responsible.