Why does a question sometimes not appear among the linked questions?

Question

I have noticed several cases when a post is linked in a question (or in an answer, in a comment, in a duplicate banner) and yet it is not among linked questions (displayed in the sidebar). I do not mean immediately - I know that it sometimes takes some time. I am speaking about links which have been there for months or years and yet they are not shown.

I will list some examples below - they come mostly from the sites I visit often. (If there is some kind of SEDE magic which could help to find other such situations, I'll be glad to learn about that.)

I have these questions:

1. Why does this happen? (Is it only in case of old questions before linked questions in the sidebar were implemented? If age of post/comment can be a factor, when did this change? In the case of MathOverflow, can this be caused by migration process to SE network?)
2. How can this be corrected? (Can the linked questions be updated by some automated process. If not, what could I do when I see such situation and I think that the linked question is important enough to have higher visibility and should be shown in the sidebar.)
3. What can I do to prevent this. (If this is intended behavior in some situations, how can I make sure that my link will appear among linked questions?)

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Here are some examples. I have added Internet Archive links - so that this can be checked if the situation changes.

• Mathematics question Number of ordered pairs of coprime integers from $1$ to $N$ has an answer which links to Efficient way to compute $\sum_{i=1}^n \varphi(i) $, but this question is not shown in the sidebar. (I am not sure whether it is relevant, but the link is of the form https://math.stackexchange.com/a/422477/1778.) The only question shown among linked questions is question 1929485, while the answer links to question 421274. The question 421274 does not have this question among linked questions either. Wayback Machine and also the link for 421247. EDIT: The linked questions are displayed now. But you can see from Wayback Machine that this was not the case a few weeks ago. As explained in the comment, this changed after Shadow Wizard posted a link and then deleted the same comment.
• Mathematics question How to find the sum of series $\sum_{i=1}^{\infty}\frac{i}{2^i}$? has commenst linking to Prove that for all $n\in \mathbb{N}$, $\sum_{i=1}^n i/2^i = 2 - (n+2)/2^n$ and Why $\sum_{k=1}^{\infty} \frac{k}{2^k} = 2$? yet these questions are not shown among linked questions. The comments link to 337937 and 512065, the questions shown in the sidebar are 30732 and 674220. 51026 has no linked questions. Wayback Machine and also link for 51026 This one can probably be explained by the fact that https links did not work before 2015 according to this answer.
• MathOverflow question Proof assistants for mathematics links to Intro to automatic theorem proving / logical foundations? Yet no question is among linked questions. List of linked questions of 1017 is empty, too. Wayback Machine and also link for 1017.
• The very first comment under MathOverflow question Number theory textbook based on the absolute Galois group? contains links to Galoisian sets of prime numbers and "Understanding" $\mathrm{Gal}(\bar{\mathbb{Q}}/\mathbb{Q})$. Yet no question is shown among linked questions. (And the question is not shown among linked questions in the sidebar of those two questions either.) Wayback Machine and also links to other questions: 11747 and 2791.
• Mathoverflow question How many dimensions I need to embed a graph? was closed as a duplicate of What is the max number of points in R^3, interconnected by generic curves?. Yet neither of them shows any linked questions in the sidebar, see here and here. Wayback Machine link for the former and the latter

Answer 1

Most of those are not bugs, as far as I can see. I'll elaborate.

First of all, the Linked Questions feature was added in April 26, 2010 as announced in this blog post. The feature was not retroactive, i.e. it did not go over all existing links, so any link posted before that in a question, answer, or comment, did not cause the target to be linked. (Same for closing as duplicate.)
This explains the lack of Linked Questions here, here, and here.

Second, as reported in Questions don't link if the URL uses HTTPS there was a long time bug that caused https:// links to not be picked, and it was fixed only in February 2015, and like the original feature implementation, it did not check existing links.

As for the example here this looks like a bug indeed, I was not able to find a way to reproduce.

Answer 2

I asked a question like this on meta.puzzling.

The answer was basically that it had the link as q/number instead of site.stackexchange.com/q/number(/name-of-question).

This doesn't explain all of them, but it should certainly account for some of them.