When you have <details> <summary> tags and you apply the CSS list-style: none; then the arrow should be hidden. This works fine on other browsers (e.g. Chrome, Firefox).
<rdar://problem/80527187>
Thanks for filing, if possible please attach a reduced test case or link to a live example.
Here's a live example: https://buonmenu.com/#faq The FAQ don't have any arrow in Chrome, Firefox, etc. but an arrow is displayed in Safari.
*** This bug has been marked as a duplicate of bug 157323 ***