Can not confirm

I was able to reproduce it consistently.

See this video: https://drive.google.com/file/d/1uohUTK_uFKCHwIDZcbtCEwaUBAHYRhMu/view?usp=drivesdk

It only happens if I select and want to highlight the entire word.

A selection of words works though.

I can see from that screenshot and video that you are not using the tinymce editor that is shipped with Joomla.

You're right, I am sorry. I used TinyMCE of Yootheme for that matter.

But the same applies to core Joomla.

Can you try the following:

First enter a text and a link. Then mark all text as "Paragraph". After that mark all text as "Bold". But the link part does not get bold.

I cannot reproduce. Please click Toggle Editor to see the generated HTML.

still can not replicate. Sounds like the yootheme implementation of tinymce is broken

Closing as non-reproducible in core and most likely with Yootheme's implementation.

Can you please try the following?

The same happens inside core Joomla.

See this video: https://drive.google.com/file/d/1lbi92tpxERAi1rEKYDMJ7KiI_6lWgmqD/view?usp=drivesdk

Still not reproducable

Then I have no other idea...

Thanks for testing though!

Please use browser's inspector to see if the bold styling has been removed via css on the backend.

I am not sure how to do this.

Like this?

Perfect! On my system, the font-family is san-serif, but on yours it is -apple-..... This could be the problem. Have you tested on a clean install without Yootheme?

Thanks!

Not yet.

You think Yootheme can cause this problem by somehow changing the font-family?

Can the problem the following?

Cassiopeia "overrides" the editor.css file and uses its own

https://joomla.xxx/media/templates/site/cassiopeia/css/editor.min.css

And that works fine.

But without an override Joomla loads this file

https://joomla.xxx/media/system/css/editor.min.css

So I guess the question why the two editor.min.css files (owned by Joomla) are not identical.

@Quy @brianteeman could I get your input on my latest comment?

Thank you!