[vim/vim] Changing 'isprint' breaks drawing of unrelated Unicode characters (Issue #21053)

Bricktech2000 created an issue (vim/vim#21053)

### Steps to reproduce

`vim -u NONE -i NONE` then insert U+222B INTEGRAL into the buffer (`i<c-v>u222b<esc>`). Now run `:se isp+=^226` and the integral character turns into a less-than sign. `:se isp&` and the integral is back.

With a number other than 226 the integral looks fine. The UTF-8 encoding of U+222B is `e2 88 ab`, and 226 is decimal for hex `e2`. It's almost like one part of Vim only looks at the first UTF-8 byte and thinks the character is non-printable and tries to display `<222b>` while another part of Vim correctly considers it printable and only allots one cell for the character and truncates `<222b>` to `<`.

### Version of Vim

9.1.1566

### Environment

NixOS 25.05, 'encoding' is UTF-8

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