Yeah, this is an issue in Access i came across myself. Essentially, long text fields don't have scroll bars (Well, mine doesn't) and yes, as a result, the mouse scrolling doesn't work.

I found this article (From 2014 but should still be relevant) but haven't had a chance to play yet. Maybe it'll help?

https://www.devhut.net/ms-access-enable-mouse-wheel-scrolling-of-text-box/

I see the same results as you.  
Unfortunately I have never heard of this issue previous to now and I don't recall ever seeing any way of controlling the mouse scroll in Access.  
I'm afraid that this is probably one of those things that Microsoft has never addressed in the program and you may have to live with it or just use plain text.

I've just looked at the demo DB provided on that Devhut page (MB scan all ok) and it contains a public module which ties to the LText box on click event. It actually looks good but it's built for x86 systems so generates an error but otherwise works.

What does Rick of this?

For x64 Users, theres a comment with two lines of code to replace in the comments on the page. Just tried and it works!

Dan if you are asking me - I would say that he probably would pass on it or figure out a solution of his own.  I know - based on years of watching his videos - that he is not in favor of using 3rd party solutions due to compatibility issues.

If you are asking him specifically - We'll see what he says when he sees this thread.

Thanks for all of your help guys! I think I'm gonna stick to plain text then.
...unless Rick comes along and shows off one of his magic tricks to solve this one of course ;)

I've never even heard of this problem before! I'll do some research and get back...