Hi Olga,
Yes, this occurs in the latest version (8.0.2403.26). When the file is first loaded the external subtitles show, but as soon as an edit is done they disappear. If the saved project is loaded, the subtitles appear briefly but then disappear. Here is your link to the uploaded files: https://drive.google.com/drive/folders/1RHcQp7u_Kg0i7DqB0d9mi6gzhuT7nvhw. This file is only an example of what occurs frequently with other files I have recently edited. If I use MKVToolnix and combine the files, the (internal) subtitles continue to show in the preview window during editing.
Thank you,
Dwight