What are the allowed values for these, can that be added to the doc, please?

Also, regarding the disappearing underscore cursor in 3.17 list items in my case at least, this was because I was using Jetbrains Mono and line height of 1.0 (instead of default 1.2).
When I update my screen size to 50x160 from 62x160, giving room for a line height of 1.2, I can see the cursor.

I would like to get some feedback that this is a useful way to resolve the reported issue. Then I can also update doc as part of this PR before it gets merged.

You could try ZTI_CURSOR_REPLACE=5 before running zti to resolve. Zti will use any positive integer you give it. Which values will be effective will depend on the terminal emulator you use. Values 0 through 6 should usually work - see the URLs I put in the description.

@v1gnesh, that is very interesting that the size of the font resolved your issue!