1st comment.
You can at least validate the correctness in terms of calculation acfuracy by comparing your results between the two approaches, as the assumption must be that they would be the same whether you wee a speed benwfit or speed regression. Probably its wrong because you lost the groups. Probably ungrouping is a mistake in so far as it wasnt replaced by an alternative that can chunk your data into the required groupings. Possibly nesting or splitting would be the way to go.
2nd comment is a standar one that i will reproduce aboit the benefits of making a reprex that should make your issue demoable by forum users to help them to help you with it.
Thanks for providing code. Could you kindly take further steps to make it easier for other forum users to help you? Share some representative data that will enable your code to run and show the problematic behaviour.
How do I share data for a reprex?
You might use tools such as the library datapasta, or the base function dput() to share a portion of data in code form, i.e. that can be copied from forum and pasted to R session.