After clicking on the image link icon the field where you add the link appears at the bottom of the image, and if it's a large image you think the field has disappeared but need to scroll down to see it. I reported this as a bug years ago
If it is the same issue, my bug report is here (re:alt text)
This issue happens when there is a small browser window and/or large image so that the message text box has vertical scroll bars
1 Insert image to post
2 Select image to view image edit buttons *
3 Click on i to add alt text
Result: No text field for alt text.
Workaround is to
Scroll browser back to see the image scroll bars
Scroll message browser to see bottom of image
Highlight image again to see the image edit menu.
Note: this is even more clunky on mobiles.
* Why is the indent/list dropdown showing over the image ? I...
In that case it was the field for alt text but possibly the same issue. It is noted as 3rd party, presumably that means the Froala editor which we know is very buggy and going to be replaced in version 3 of xenForoi