Hi Manux23,
I'm still using this in WordPress 5.2.2 with success. Maybe my explanation wasn't clear.
Note: did this work properly in WordPress 5.1 on your website?)
So in my case, I have created my own theme from scratch.
In the theme code (in functions.php), I call the function "get_comment_reply_link()" to get the proper link for posting a reply when building the reply form.
Note that not every theme will do this, some will actually use the built in function instead (in that case, this trick will not work - and you may have to contact the person or company that created the theme you're using, asking for an update).
My original form code looked something like this, and the code for your theme may be different;
$replyButton = get_comment_reply_link(array_merge( $args, array('add_below' => $add_below, 'depth' => $depth, 'max_depth' => $args['max_depth'])));
// potentially some more code here
echo $replyButton;
As of WordPress 5.1 this no longer seems to produce the correct link. It would produce something like this (in the HTML of your page):
data-belowelement="comment-form-here-212086" data-respondelement="respond"
There used to be (if I recall correctly) an "onclick" JavaScript event, something like this:
data-belowelement="comment-form-here-212086" onclick="comment-form-here-212086" onclick="return addComment.moveForm( 'comment-form-here-212086', '212086', 'respond', '178' )" data-respondelement="respond"
(the bold part of the code is missing).
I'm sure there are better ways to fix this, but I was looking for a quick fix (getting a little tired of making fixes for functionality changes I didn't ask for - and frankly a little disappointed that the WP team is not really paying attention to backwards compatibility).
Anyhoo, I used "str_replace()" to "add" the missing code, which resulted in:
$replyButton = get_comment_reply_link(array_merge( $args, array('add_below' => $add_below, 'depth' => $depth, 'max_depth' => $args['max_depth'])));
// potentially some more code here
// WP5.1 fix - missing:
$replyButton = str_replace( 'data-belowelement', 'onclick="return addComment.moveForm( \'comment-form-here-'.get_comment_ID().'\', \''.get_comment_ID().'\', \'respond\', \''.get_the_ID().'\' )" data-belowelement', $replyButton);
echo $replyButton;
If unsure, you could try to see if you can retrieve what the reply button link HTML code looked like before updating to 5.2.2, and then compare it to what it has become after the 5.2.2 update.
But I understand this can be tricky, since WordPress never offers an easy way to revert back to a previous version of WordPress. In my case I got lucky, since I had upgraded WordPress first on a copy on my computer running for testing and development. This is where I can into this issue and could still retrieve the missing info from my real website.
What you could do, if you do not have access to the previous version, or are nervous about move back to a previous version (I would), then you could open the affected page in Google Chrome or FireFox and open the developer tools, so can see and edit the code. Now you can modify the code of one of the reply buttons and change it to the full code with the onClick event code added, to see if it works - even before modifying the functions.php code.