I don’t see this as often on iOS as on Android, but I’ve seen this happen several times on both OSes; both here on Meta and on the NaNoWriMo forums as well.
I also see it really rarely on a selt-hosted install that has no plugins or theme components. In fact the only customization it has is it defaults to using the WCAG Light color scheme as the default for the Default theme.
I may have a small lead on this particular one. Could it be when you’ve opened the Reactions menu on mobile and then click away (without selecting anything)? After that, it seems post text is unable to be highlighted.
The fix, which was mentioned in the second post, will be included there. But there was a report that this fix did not help here.
The issue@JammyDodger described, which is related to the reactions plugin, is not fixed. But with this issue you cannot highlight text at all, so it is different from your issue.
So all in all the issue was fixed for some sites. Even if there is another fix, you won’t know whether your bug was fixed untill december, because you at the stable branch you only get security fixes between releases. That’s the advantage of tests-passed you can update directly after a fix is there.