xtof54 is a user on octodon.social. You can follow them or interact with them if you have an account anywhere in the fediverse.
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I think I've just understood something about #mastodon + #api: when you *reply* to an external toot, your reply just appears in the target Home TL; further adding @mention makes it also appear in the Notifs TL. But your reply is not listed neither in source nor target local TLs !?? Got to rethink my app then... :-/
This is pretty bad, because you cannot have public "replies" then ?!