JSON strings are not parsed properly by HTTP Capacitor core plugin. See discussion here: ionic-team/capacitor#6170 (comment)
This app reproduces the error. The following valid json
"hello world"is fetched from this url https://run.mocky.io/v3/cb25a9f8-1ef5-4462-8e5a-fc56de26b876
Run it in browser with npm run start and it shows the text hello world.
Run it on iOS or Android and an error is shown.
Here is a PR that remedies this problem ionic-team/capacitor#6224