Independently from tdf#155823 mentioned by @mikekaganski (which I didn’t study in detail, having just discovered it), your initial .odt is structurally-wise faulty. This may be the cause of the apparent corruption when converting to .docx because numbering (list and chapter, all the same) is one of the most delicate compatibility issue.
The problem stems from the definition of a list. What makes a list specific is its list style. Most of the time Writer users create list with Format>Bullets & Numbering (or its toolbar equivalences) where they control nearly nothing (from an abstract typographical point of view). This approach was provided as a “compatibility” bridge with Word where the concept is a real mess. Since this is compatibility feature (most users qualify it “intuitive” while it is not) tries to deal with not-otherwise-advertised lists (you don’t specify where it starts nor where it ends), Writer must make guesses trying to rely on its strong theoretical basis. And what user did does disturbs this guess-party.
First of all, users don’t realise that all lists in Writer are multi-level with a huge versatility in the numbering scheme (practically all levels are independent from each other). Consequently, when a nested list is needed, a single list style will do.
Second, and most important, a list style associated with a paragraph will define the “type” of a list. All paragraphs with the same list style will be part of the same list. Of course, you can split this “type” into several instances by restarting numbering. The list style characterises a list type and gives this type its homogeneity and personality.
Your .odt is totally direct formatted. By itself this already results in formatting nightmare. But when lists are involved, this turns into formatting hell.
You configured your first level-1 to be numbered with the use of the button in the toolbar. This associated your 1. and 2. items to Numbering 123 list style. When you typed your first level-2, you again numbered it with the toolbar button, choosing alphabetics. But this associated items to Numbering abc.
This is obvious in the XML snippet you show in your question.
Consequently, instead of having a single nested list, you have two independent lists: one with numbers where only level-1 items are present, one with letters where only level-2 are present. These lists are intermixed, which is allowed but gives surprising results for incautious users. Level-1 numbers in the second list are not synchronised with those of the first list.
I don’t know exactly how you typed the second alpha-item, the one listed as b, but it may have incurred another hazard of the kind which detached it from the list “type” of the a item.
When you original source document is not consistent, don’t expect conversion to alien format where theoretical foundation is not assured (such as .docx) to perform smoothly.
List numbering (and in a lesser extent chapter numbering because the choice is more restricted) is a complex feature which is not clearly explained in the official documentation. It is exacerbated by direct formatting (which unfortunately most users consider easy and intuitive). The only reliable approach is styling. But it requires some thinking ahead. You can’t dive into writing your document, unless you type just as you would do with a text editor, i.e. absolutely no formatting. Formatting would be added at a later stage with styles.
At this stage, without deep analysis of tdf#155823, you’re the sole responsible for the erratic conversion to .docx. You should structure very rigorously your documents. Then if they don’t display or convert as intended, ask for help here.
