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Update: The long-running process finished. The terminal prompt still shows the same mangled output. running echo $LANG (or ␊␌␤⎺ $LANG, as the terminal shows it) produced ␊┼_US↓UTF↑8, so seemingly still en_US.UTF-8.

running reset (aka ⎼␊⎽␊├) restored the session's text to normal, so for the moment the problem is resolved. I'm still interested in understanding exactly what happened, and how I can resolve it without waiting for the process to end first if it reoccurs.

• Both the server and my local devices should be using UTF-8, with the language set to US English (echo $LANG gives en_US.UTF-8).
• The program is running in Python 3, and using normal print() calls to output to the terminal.
• ANSI escape sequences for color codes are behaving correctly, all the output is colored exactly as expected.
• The program is printing a mix of hardcoded information but also text from documents it reads, and those documents may contain unusual or invalid unicode sequences, which is likely what caused the problem

• Both the server and my local devices should be using UTF-8, with the language set to US English (echo $LANG gives en_US.UTF-8).
• The program is running in Python 3, and using normal print() calls to output to the terminal.
• ANSI escape sequences for color codes are behaving correctly, all the output is colored exactly as expected.
• Only the one tmux pane seems to be affected, all the other tmux panes around it are still completely normal and usable.
• The program is printing a mix of hardcoded information but also text from documents it reads, and those documents may contain unusual or invalid unicode sequences, which is likely what caused the problem

Edit to add some details based on questions in the comments:

• Both the server and my local devices should be using UTF-8, with the language set to US English (echo $LANG gives en_US.UTF-8).
• The program is running in Python 3, and using normal print() calls to output to the terminal.
• ANSI escape sequences for color codes are behaving correctly, all the output is colored exactly as expected.
• The program is printing a mix of hardcoded information but also text from documents it reads, and those documents may contain unusual or invalid unicode sequences, which is likely what caused the problem