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By the nature of the scenario, there is a chance it’s an isolated scenario that is present in your environment. To get the root, if possible check by yourself by mentioning those mention who are not getting the link. As you mentioned there doesn’t seem to be a pattern, I believe you have done that already and you made sure that there aren’t any spaces, and such a scenario.
Now, you can check theme/plugin conflicts. Although there is a very low chance that there would be conflicts still. To do that, you can try this plugin: https://wordpress.org/plugins/health-check/ Also please consider checking WP-Admin->Settings-> Discussion to see where there is any moderation for those users.
If the above are all good, consider checking whether there is any location or any other restrictions from the server side. I haven’t faced such a scenario as well, that’s why, I am curious as well to know what exactly is root cause.
