Okay, messages are received from myself which previously were not. The new behavior was extra confusing to me that these kept appearing every time I connected - but this was because of my scripts (or rather - the script needed to do more). I was already sending a clearbuffer message to znc every time I received a message. This took care of the playback from other users. But when I received these messages from myself, I processed them as expected but did not issue a clearbuffer (because I had not needed to in the past). So I'll issue that command now and that'll be fine.
The only other problem then is receiving back messages I *send* which I believe is what you're referring to in your edits and the other filed report(s). Sending messages to myself means that the scripts process on the first received message and also on the echoed copy of sending it. Duplicated messages also appear in the playback, if you handle that separately.
<- @time=2022-05-12T21:01:27.049Z :Membear!Membear@cloak-x96fe2.com PRIVMSG Membear :this is a single message
-> irc.server.net PRIVMSG *status :clearbuffer Membear
<- @time=2022-05-12T21:01:27.090Z :Membear!Membear@Membear.vhost.Net PRIVMSG Membear :this is a single message
-> irc.server.net PRIVMSG *status :clearbuffer Membear
<- :*status!znc@znc.in PRIVMSG Membear :1 buffer matching Membear has been cleared
<- :*status!znc@znc.in PRIVMSG Membear :0 buffers matching Membear have been cleared
