It seems like YouTube is removing the skip button for the portion of the ad that isn't skippable anyway. Typically, skippable ads must run for a specific duration (five seconds) before the user can pass it off. The change on mobile devices shows a full bar for the ad's supposed "duration," which is likely just the portion users must see.

YouTube is testing major changes to the skip button that seem to make it more difficult to find, so you might end up watching more ads.

YouTube appears to be testing a new method to prevent users from skipping ads by obfuscating the Skip button. The change seen by multiple users involves adding blackout squares over ads to hide the Skip button, potentially as part of a limited test.

YouTube's potential removal of the ‘Skip’ button raises concerns about user experience amid increasing ad segments.

Android Police reports that YouTube has removed the countdown for skippable ads, making the skip ad button appear abruptly on mobile.

YouTube ad segments can be annoying but have a Skip option available after a few seconds. Users have noticed a change in the Skip button's display, potentially leading to confusion. The absence of a countdown timer in the Skip button's usual spot seems to be intentional.