@Tomas Blažauskas Thanks for getting back, I had a discussion internally and confirmed that this version mismatch will have no impact at all.
The reason why the portal is not displaying the expected version ".NET 8 Isolated" is because the site property ".netFrameworkVersion" seems to be still set to "v4.0". Our tooling sets the value when the function app is created. However, if, for example, an ARM template is used to create the function app, and the property is not set in the template, it defaults to "v4.0". Most likely this could be your scenario.
do let me know incase of further queries, I would be happy to assist you.