Seems like a bug. Here are two other open issues for your reading pleasure 
- [IOS] Cookies problems in IOS 路 Issue #1373 路 ionic-team/capacitor 路 GitHub
- bug: 3rd party cookies not working on capacitor iOS 路 Issue #6302 路 ionic-team/capacitor 路 GitHub
Can you use signed URLs instead? That鈥檚 what we use in our app 
Relying on Cookies in a native app shouldn鈥檛 be your first choice. There are so many posts here with people having issues with Cookies, if you can get around not using them, you鈥檒l thank yourself later.