Update label as the value is changed (dynamic update?)

1. What is the problem? Be very detailed.
I am trying to produce a range question for a 'ratio' choice between two options that works around the need for numerical range, rather than the text showing on the slider - see Slider widget with text as begin and end label

I thought I'd come up with a rough-and-ready solution which dynamically updates the label as you move the slider. I did something similar using a likert select_one widget for comparison, using the same 'trick' as that was a suggestion in the previous post / suggestion.

This is my form (two examples as above in a field-list group - because in the real world form I intend to use that, so I want to check that it will work!). I uploaded using XLSForm online, and previewed (Enketo) - it worked exactly as I hoped... as soon as I move the slider, the label changes.

ratio_example.xml (2.1 KB)

However, when using the form on my phone the labels on the question are static, until I swipe past the group, then when I swipe back they have changed to the expected values.

Is there a way to force the form to update the label on the question you've just modified? I did look at this discussion: XLSForm proposal to calculate a value when another value changes but I'm not sure it addresses updating a label?

2. What app or server are you using and on what device and operating system? Include version numbers.
Collect v1.28.1

3. What you have you tried to fix the problem?
I added a 'note' field with a duplicate of the label from the range question and this works. It updates immediately that the value in the preceding question changes. So I've sort of solved the problem (again I've worked around it, rather than solved it!), but it's not a 'clean' as using a single question as it has a bigger screen footprint...

4. What steps can we take to reproduce the problem?

Follow what I have done / load the form to your device.

5. Anything else we should know or have? If you have a test form or screenshots or logs, attach below.
Here is the XLSX file for what it's worth - it sort of works, but is visually clumsy at the moment.
ratio_example.xlsx (9.6 KB)

You could change the 'step' in the range to give a finer-grained response if that's important.

Real world use: We have a vegetation quadrat and are asking farmers to tell us the proportion of grasses compared to other plant types (forbs) - it needs to be easy to use and avoid them getting hung up on exact percentages of each thing (at this stage!). A slider seems to be a simple solution and this is the nearest I can get at present.