@33rn thank you for the information, that makes sense.
I have one continuation question for that: since I asked that I've closed less tabs than I have opened from other applications (mostly from my email app), so does that mean I now have more than 99 tabs, or does some previously opened tab get overwritten? I've never had Opera Mini refuse opening a link from some other application.
Edit: nevermind, I got my answer when I closed one tab and still couldn't open new ones - I then tried opening 2 more from K9-Mail, and it worked. Apparently I've gone over 99 by several tabs now - time to close some down 😜