There's a known issue for this that came about after the introduction of Enable Unread/Read on Compact Case Feed in Winter '20.
EmailMessage trigger gets invoked when viewing the message in feeditem
Interestingly, this has been requested as a feature for a while - but, this is only happening when viewing the email message through the feed. It's actually firing when you click on the info icon (or the expand icon) in the screenshot below (apex trigger and record-triggered flows).

You can workaround this by adding an entry condition in your record-triggered flow to not fire when Status
changes on EmailMessage since that is the update that is occuring - changing from 0
(New) to 1
/(Read).

Otherwise, you could set it to only run on create if you don't actually need the update condition as it's not clear, from your question, if you are doing anything in that scenario.