Same issue, as @mraassin. Turned out to be the Yoast SEO plugin for me, tablet and mobile views work as they should with the plugin disabled. I posted in the Yoast SEO forum: https://ww.wp.xz.cn/support/topic/tablet-mobile-view-in-block-editor-disappears-wp-6-6/#new-topic-0.
Hello Suwash –
Thank you for the reply, that worked, thank you for your time.
Lukas
