Why is this F# sequence function not tail recursive?

Disclosure: this came up in FsCheck, an F# random testing framework I maintain. I have a solution, but I do not like it. Moreover, I do not understand the problem - it was merely circumvented.

A fairly standard implementation of (monadic, if we're going to use big words) sequence is:

let sequence l = 
    let k m m' = gen { let! x = m
                       let! xs = m'
                       return (x::xs) }
    List.foldBack k l (gen { return [] })

Where gen can be replaced by a computation builder of choice. Unfortunately, that implementation consumes stack space, and so eventually stack overflows if the list is long enough.The question is: why? I know in principle foldBack is not tail recursive, but the clever bunnies of the F# team have circumvented that in the foldBack implementation. Is there a problem in the computation builder implementation?

If I change the implementation to the below, everything is fine:

let sequence l =
    let rec go gs acc size r0 = 
        match gs with
        | [] -> List.rev acc
        | (Gen g)::gs' ->
            let r1,r2 = split r0
            let y = g size r1
            go gs' (y::acc) size r2
    Gen(fun n r -> go l [] n r)

For completeness, the Gen type and computation builder can be found in the FsCheck source