Hey Squeezefan,
Does this happen when using either of the tab layouts; Single Tab or Tab Bar?
Change the layout in Safari on iPhone
In the Safari app
, you can choose the layout that works best for you. Depending on the layout, the search field appears at the bottom (Tab Bar layout) or top (Single Tab layout) of the screen.
To choose a layout, do one of the following:
• In the Safari app, tap
in the search field, then tap Show Top Address Bar or Show Bottom Tab Bar.
• Go to Settings
> Safari, then scroll down to Tabs. Select either Tab Bar or Single Tab.

Change the layout in Safari on iPhone - Apple Support
If the other tab layout does not reopen closed tabs, change back to the original option, restart your iPhone, and please test again.
If the issue still happens, or if it happens with either tab layout, the next step is to contact Apple Support, to let them know what's happening.
Apple Support
Take care!