iPhone safari tabs keep closing

Since the last update of 3 days ago, when I close the Safari browser on my iPhone and then reopen it, all the tabs in my main tab group are closed. The tabs in the smaller tab groups are still open. I don’t have private browsing on. This happens whether I have 30 tabs open in the main group or 2 tabs open. I have 58 GB of 64 GB used. I’ve cleared my website data in Settings. My battery still has decent life, lasts several hours with mild use. It’s an iPhone 8 Plus, software version 15.5 (I know, I know, it’s old, but it works well and still gets updates.).

any suggestions?

Many thanks!