Subject doesn't changed after the editting

[Sulpher]

Describe the bug

A topic has a few replied.
The subject name is: testing the feature 2

I decided to edit the subject of the last message and changed its name to:
testing the feature 333

But... The subject was not changed on the topic page.

Here are the screenshots illustrating this problem:

As you see, I added 333 to the topic name.

After clicking on "Save" button, I see the topic page and my edited message still has the old subject:

But if I edit this post again, I see correct topic name:
testing the feature 333

System information (please complete the following information)

Joomla version: 3.8.13
Kunena version: 5.1.5
Crypsis BS3
PHP: 7.0.32

[sshcli]

This issue is related to this one: #4283
It doesn't mean it's duplicated.