What LinkedIn says about comment sorting
LinkedIn's help page says comments can be sorted by Most relevant or Most recent using the dropdown below the comment box, and that some comments will only be visible under the Most recent view. You can check it in the official help: Comment on posts and reply to a comment.
In practice, two readers opening the same post can see comments in a different order, and some comments may only appear when you change the sort. That is why your comment can look missing when it has been published.
What is documented, and what is not
LinkedIn does not publish the criteria that define how relevant a comment is. Everything circulating on the topic (ideal length, word counts, the weight of replies) comes from observation or guesswork, not a confirmed rule. Do not build a strategy on a precise formula.
What is reasonable, though, is writing comments that add something to the post: an example, a nuance, a precise question. That helps readers whichever sort they use. For the wider context, see our entries on the LinkedIn algorithm and dwell time, and the LinkedIn glossary.
When to use Most relevant or Most recent
Most relevant suits you when you want to read a post's discussion without scrolling through dozens of replies: you see first the comments LinkedIn surfaces for you, without knowing exactly why. Most recent is more useful in three cases: finding a comment you just posted, following a live conversation, or replying to people who have just joined in.
For the author of a post, this has a practical consequence. When you reply to your comments, switch to the Most recent view from time to time so you do not miss a message that does not show in the other view. Replying to everyone who took the time to write is a good habit, whatever the sort.
For a comment you write yourself, the best practice stays the same: read the whole post, respond to its content rather than only to the author, and avoid generic lines like "Great post!" that say nothing about the topic. A precise comment gives other readers a reason to stop and reply.
Last, remember that the sort you see is a display choice for you as a reader. It does not delete anything: every comment stays attached to the post, and switching the sort is always available if something seems to be missing.
If you want quality comments on your own posts from professionals who actually read each other, you can try Podavera, the LinkedIn engagement exchange.