You do know that there is current / flow inside the display? A weak fish could easily be swept into the anemone.
I don't know what sort of anemone you have so won't comment on it other than it isn't out of the realm of possibility that your fish died from other causes. Usually aggressive anemones will eat the fish. Not leave it or hold it.
Regardless you should assess your display, fish, and decide what you want to do moving forward. You could send off an aquabiomics test and see if it reports any pathogens. That takes up to a month though so not immediate. While you wait for that don't add anything to the display and decide what you want to do with the anemone.
If you keep it then your fish selection may be reduced. Rehome it and wait. At a minimum I wouldn't add anything to that display for 3 to 5 months until you have a better estimate as to what happened. I wouldn't just assume it was the anemone. But that is just my opinion.