Landing Page (not set) up 4x after Google Analytics moved to GTM

We normally have a small percentage of landing page (not set) sessions due to some events we have set up on the homepage (scroll depth tracking, click tracking) - most likely from sessions that expire due to inactivity and are then reactivated with the scroll or click that triggers the event in Google Analytics.

At the beginning of the month, we moved Google Analytics from the website's source code into Google Tag Manager. Since then, the number of sessions (not set) has increased by around 4x-5x. The source / medium data is still there.