"Next Comment" jumps to a random comment

I have a lot of comments on my prototype.
When I open a comment, there are two arrows (next & previous) which can be used to cycle through the comments without having to open & close each one individually.

HOWEVER, when using the NEXT button it seems to jump to a random comment. I would expect it to jump from #1 to #2 to #3… but it just jumps all over the place randomly #2 - #17 - #12

how do I get comments to index in a sensible order ?