When adding and editing products with ampersand (&) in model field, it will be changed into & when saving.
On the same product you can add ampersand in title etc., and it's saved correctly there (as &).
So it shouldn't be a charset issue for the site, as it's isolated to the model field.
Zen Cart 1.5.5e
PHP 5.6.32
UTF8 (utf8_unicode_ci)
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