> 'Passt übergroße ...
> mySQL error: Incorrect string value: '\xFCbergr...' for column 'sub_title' at row 1
Good info. Now, notice that hex FC in
http://mysql.rjweb.org/doc.php/charcoll#8_bit_encodings
is the 'ü' of 'übergroße'.
This says that the FC is latin1, not utf8.
Plan A:
Do "SET NAMES latin1" or mysqli_set_charset('latin1'); that will tell the connection that your data is in latin1, and to convert (if necessary) to/from the datatype associated with the column being INSERTed or SELECTed.
Plan B:
Arrange for the data you have ('Passt übergroße ...') to be in utf8 encoding.
We still need to see what got into the database table. I suspect that this string was truncated to 'Passt ' as it was inserted.
The German sharp-s has a shaky history. See
http://mysql.rjweb.org/doc.php/charcoll#german_sharp_s_
(That section is probably not important in your case.)
I'm sorry that that web page is so long. I have not figured out how to make it both shorter and applicable to everyone.