Mobile-only drops with desktop flat almost always come down to one of these — worth checking in this order:
1. It might not be a real drop. Average position is an average across every query you appear for. If you started showing up for a bunch of new long-tail queries ranking at position 30-40, your average tanks even though nothing you already ranked for actually moved. Check clicks and impressions alongside it in GSC — if clicks are steady or up, you probably just gained reach. Don't panic on avg position alone.
2. SERP layout, not your ranking. Mobile SERPs pack in way more above the fold — AI Overviews, People Also Ask, ads, local pack. Same organic rank, but it's pushed further down, so your effective/measured position slips on mobile while desktop (more breathing room) holds.
3. Page experience. Google leans harder on mobile signals (mobile-first indexing). Pull up Core Web Vitals in GSC and split by device — a mobile LCP/INP regression around your date is a prime suspect. Also check for intrusive interstitials/popups that only fire on mobile.
4. Timing. April 19 lines up with the tail of recent core/spam update volatility for a lot of people. If it's update-related, you wait out the rollout before reacting — chasing it mid-rollout usually backfires.
How to actually diagnose it: In GSC → Performance → filter Device = Mobile, compare the 28 days before vs after April 19, then Compare by query and by page. If it's a handful of pages → on-page/CWV issue. If it's sitewide and uniform → SERP layout or an update. That'll tell you which of the above it is in about 5 minutes.
What's the niche, and is it specific pages or across the whole site?