What monitor do you have plugged into your Mac Pro, and how?
I was having no end of problems with RAM preview and After Effects. I discovered that After Effects RAM preview has issues if one of the connected displays is running at other than 60 Hz.
Go to “About this Mac” and choose “System Report”, then choose “Graphics/Displays” under the hardware menu. If any of the displays say they are running at less than 60 Hz, you need to fix it.
In my case I had a 4k Vizio television plugged in via HDMI, and unbeknownst to me it was running at 30 Hz. I had to get a different USBC to HDMI adapter, and use a program called SwitchResX to create a custom resolution to get it to play in 2k in 60Hz. The TV apparently doesn’t support 4k at 60Hz, even though it was advertised as a 4k 60Hz tv. It was one or the other.
After I got my display running at 60Hz it solved all sorts of RAM preview issues that I had been grinding my teeth over for months.
-Matt
Check out my plugins for AE, Premiere, Motion and FCP.
https://monadnock.org/plugins/