Hello @GrahamRoberts
I tried that Flow and get the solution !
You can solve it by adding just one step in the flow you have to Convert the HTML to plain text via just one step before you Update the task details see the Image below
You have to put the value of the email body into the converter for to convert the HTML text to the Plain Text.
(In my case i have Taken the Body of the Email, In your case you can take the "Event details" i have Double checked i assure you that it will work on both ways on Email Body Content and in Sharing list Field too.)
at last use the plain text Dynamic Content where ever you want to use.
(In my case i just used the Only plain text Content but in your case you have to replace it with the Event )
details
Thanks