Manufacturers always say never to mix inks unless they’re selling you a range of mixable inks, yet I have never had a problem when refilling pens that weren’t 100% clean. What are the worst reactions you ever got from mixing inks on purpose or by accident?
Manufacturers usually have to cater to the lowest common denominator.
There are very good chances that there is a large group of people who have mixed the wrong kinds of ink together, damaged their pens &/or possessions then contacted them to complain & demand compensation.
99% of the time there should be no issues but with several thousand different inks they can’t test every combination.