Hi denises0
welcome to th Apple (user-to-user) Community.
Re: "dropdown won't collapse when tabbing away"
Thank you for sharing your experience with the community.
If you are still using iOS 11, as mentioned in your post:
Seems the iPhone 7plus could be upgraded to the newest compatible iOS,
to see if the difficulty is solved. (Eg: iOS updates often contain bug-fixes, and more.)
About iOS 15 Updates - Apple Support says the latest version is iOS 15.8.2
Currently, an iPhone 7 Plus can not be updated beyond iOS 15.8.2
It is wise to back up everything on your device, before updating or upgrading.
If needed, see: How to back up your iPhone, iPad, and iPod touch - Apple Support
If you want: you can go to: Settings > General > Software update:
There you can see if your iOS version is up-to-date or not, and / or choose to upgrade.
If you have updated / upgraded to iOS 15.8.2 already, and are experiencing this issue, you may want to send your well-explained feedback to Apple. They will likely not reply, yet if others are also reporting similar issues, they may be addressed in a future update.
Web-form for Feedback - iPhone - Apple
All the best :-)