@Michael Vogel Sorry for the delay in answering you, but I did a series of tests and these tests didn't give me the results I expected. Thus, by dint of doing other tests, I finally understood what was the problem that determined "untitled" visibility in my Friendica posts with title/subject.
Clue
Almost all of the posts with title/subject that I produce are intended for publication on Lemmy and upon closer inspection, I found that ONLY the posts intended for Lemmy have this visibility problem...
Discovery
Well, I understand that the problem lies in the mention of the Lemmy community! As I should have expected based on Friendica conventions (which in the meantime I had completely forgotten) the mention of a group/forum is considered "Public" if done with the @ at sign, while it is considered "Unlisted" if done with the exclamation mark !
Since the mention of the Lemmy communities usually takes place with the exclamation point, this is the reason why the posts did not result in "public" mode but only in "unlisted" mode.
Solution
Actually, it is also possible to mention Lemmy communities with the @ sign, and in this way the produced posts are displayed as "public"
Example:
1) Standard visibility on Mastodon of titled posts posted by Friendica
2) Standard visibility on Mastodon of titled posts posted by Friendica (with mention on Lemmy)
3) Standard visibility on Mastodon of titled posts posted by Friendica (with mention @ on Lemmy)
Perhaps integrating that feature will solve this problem as well.
With Lemmy, however, that doesn't happen.
To your knowledge, was the problem presented to the developers of mastodon?