We are still having this issue, been going to all the latest releases.

I have an end user that has noticed a pattern though. If you always double-click on the e-mail to open it in a new window before hitting reply or forward, the below pattern is consistent:
If you type the message quickly, the arrow will show up. If you don't get the message typed within certain seconds maybe 20 to 30 seconds, or if you move away from the window (making another window take the focus) it will not show the arrow.

 

Hope this helps

I have a possible pattern regarding the "timing out" trend I noticed when trying to achieve a reply arrow to an inbox message.

 

So far:

 I have first been double clicking on the message (not using the visible reply arrow near the top of the e-mail screen).

 As I was typing or adding attachments I would notice a "saved as draft" bubble pop up in the lower right corner. It appears that may be right around the breaking point for the reply arrow to be activated. I'm curious if the program saving it to a draft makes it not trigger the arrow.

This has gotten fixed, I forgot what version it was. This can be marked as resolved.