Boldbeast Recorder uses a database to save information of recorded calls, this makes searching operation very fast even if you have more than 10000 saved calls.
For example you recorded 1000 calls.
You can use a file manager to see 1000 sound files in the saving folder.
In the database it has 1000 rows to save related sound file names, date, time, duration, incoming/outgoing types etc. You can see 1000 calls in the Call List screen inside the app.
In the Call List screen if you delete 10 calls, the app will delete 10 rows from the database, and delete 10 sound files from the saving folder, it's OK.
If you use a file manager to delete 10 sound files in the saving folder, the result is - you'll see 990 sound files in the saving folder but 1000 calls in the Call List screen. When you click a deleted call in the Call List screen it fails to play back of course. So you should avoid delete files by a file manager, instead please delete files in the Call List screen.
Now let us see your problem.
Boldbeast Call Recorder unexpectedly stopped working
=> No. Indeed the app works perfectly.
They are listed / visible within the app interface, but clicking on them does not play them
=> This indicates all calls were recorded and saved correctly, but you had deleted these 3 days files by a file manager, or by other apps.
the files were not visible within the MTP file/folder view either
=> This is obvious.
Is there some temporary space where I can retrieve these from?
=> All recorded files are saved in the saving folder, no "temporary space".