Hello, Saguaros!
I put anchor: <a id="anchor1"></a> inside article and made a relative link with this anchor: #anchor1.
I created the same article for Cassiopea. Now I have 2 articles for comparison:
https://allcrete.ru/allabout/photos/test-page for T4-bs5-blank
https://allcrete.ru/allabout/experience/test-page-2 for Cassiopea.
With Cassiopea anchor links work correctly.
UPD. Hm, for the experiment, I added the 3rd anchor link like this: allabout/photos/test-page#anchor3. It works with T4-bs5-blank. But the JCE editor offers the default option: #anchor3.