I agree that there now seems to be no provision for varying the length of each mark, which is almost essential on a curved carriageway. (Although the now superseded TSM Chapter 5 of 2003 said to use shorter marks only to indicate a shorter than normal controlled area – not just for a curve.) Schedule 4 Part I paragraph 5(1)(a) of the 1997 Crossings Regs allowed each mark to be reduced to as short as 1 metre if necessary.
The general tolerance on markings allowed in Reg 7 of 2016 TSRGD only permits these marks to be between 1800 and 2400 mm long. Perhaps in future we will be increasing the lengths of marks on the outside of a bend, rather than decreasing them on the inside.
I think all the omissions discussed in this thread were totally inadvertent. That accounts for PV 3 implying that the marks might be different lengths, as you say. It might have something to do with the lawyer working on 2016 TSRGD changing part way through its development. The omissions might become apparent to DfT when they write the section on pedestrian crossing markings in the forthcoming TSM Chapter 6.