I saw a few others posting about this on reddit, with no solution. I usually try to keep my open tab count low on my phone, because this happened one time maybe a year ago. My recently closed tabs only shows those from this current browsing session. From my pc, synced tabs only shows the few that are still left open. I don't have the Inactive Tabs option switched on. My pc's open tabs are unharmed (phew). I DID clear cache and cookies on my pc earlier today, but that shouldn't close tabs on another device!!