It would be good to see the bugs addressed, but it strikes me that some context around my use case - what I'm trying to do - might be helpful.
I'm trying to provide help text or guidance: additional context to the user through the label, so that the user provides more helpful feedback. I'm also intending that style of more descriptive label to personalise the form a little more.
One problem with the approach is that the resulting html element name becomes very long (e.g. name="g245-ifyoudbehappyforustocontactyoutoaskyoufurtherquestionsoranswerquestionsyouhavepleasecheckhereandprovideuswithyouremailaddress"), which I suspect may be part of the reason problems are occuring.
Ultimately, help text might be better supported using an additional field specifically for that purpose. I'm sure there are other references, but Google Forms is one I'm familiar with that provides a good example.
Note also that it appears as if a side effect of #1737-plugins is that some of the fields (noticeably the not required ones) may not appear correctly in the feedbacks record: I have at least one exmaple of a field not showing. I haven't done enough investigation to know whether this is a cumulative problem across the contact form, or on a per field basis.