May i know how to replicate this issue?

Perhaps you can check on this page and see is it i reproduce this issue wrongly?

http://www.diochi.8u.cz/index.php/poradna/14874-aaaaaaaaaaaaaaa

Write a "good day" in the reply and behind him enter the link (icon in the upper bar).
The link is inserted correctly, but the text "good day" disappears.

Hey there,

I am really sorry for the delay of this reply as it is a weekend for us here.

Perhaps you can check my video here and see is it I reproduce this issue wrongly.

http://take.ms/UngIr

if no, may i know which browser you use?

No, I first write a good day, then I will make a space and then I will put a link.
Such as the title "diocel" and url "http://www.diochi.8u.cz/index.php/pripravky-k-regeneraci/diocel".
However, this will disappear earlier words, "good day"

I still lost here.

Perhaps you can share some screenshot which part you referring?

In the link I'm sending a link video.
http://www.uschovna.cz/zasilka/MCNNX23SGLWL9D7U-KN6
I'm sorry it's so big, I'm doing a foreign PC and I can not make it smaller here.

It actually quite difficult for me to download, because my internet here not too fast.

Perhaps you can download this JING software and record a very short video and send the link to me?

https://www.techsmith.com/jing.html

I actually just need to take a look which part link you referring.

I apologize and send reduced.

Thanks, it seems like that was your template issue, when i switch to Joomla default Protostar template, it work fine.

Perhaps you can consult with your template provider regarding this and see whether their template got did some this kind of replacement on textarea ?

Thanks for kicking in the right direction.
I changed the editor to JCE in General setting-composing-default editor.
It also works properly

You're welcome