If you have game for 10 and an individual priced file for 11, a user that pays 11 has access to the individual priced file. The price for the "dlc" would be 1. The problem is, that a user that bought the game for 10 and later decides to get the dlc, cannot do so without buying the game again for 11. And as you have found out, if you would later rise the minimum price of the project to 15, the dlc will no longer be available to anyone that thought to have bought it. Because their payment price of 11 will not match the new individual price of 15+1 = 16.
Having two items to sell is always more elegant with two projects. The individual priced file mechanic is very bad for this. People use it anyway, because it exists.
It might once have been meant as an incentive in the pay what you want scheme. Offer a game for free, give a goodie for 3, so people that do pay, pay 3 instead of 2 or 1. Something like that.
Straight from the pricing faq
itch.io lets you set minimum prices on individual files. For example, if you're selling a game for $1 you could offer the option to get access to the soundtrack when paying $2 or more.
You can even added priced files when your project is normally free. Encourage a donation by offering additional content like levels or music. For example, you could offer a level pack for $2 with an otherwise free game.
The example already is ... very bad. If you offer a soundtrack or a level pack, make a separate project with the soundtrack and create a bundle of the game and the soundtrack, so people can buy both at once, or the soundtrack later.
There is a difference between a demo version and a free public version, so I understand why many devs chose that pricing option. But it is inflexible for future price changes. And uncertain in regards to bundles and discounts - at least I do not know how this works.