"Checked" radiobutton value is only working in Acrobat Reader when:

— First radio button value is "0".
— Radio button values are consecutive integers.

https://github.com/danfickle/openhtmltopdf/issues/547 — here I spent several hours to identify this issue.

Here is this post with the layout — https://community.adobe.com/t5/acrobat-reader/radiobutton-checked-value-bug-works-in-all-pdf-viewers-except-acrobat-reader/m-p/11410081?page=1#M70273

Result pdf:

See the file attachment.

Displays in different viewers: see file attachments.