I am considering reinstalling High Sierra from Mojave. I have a good copy of High Sierra and never tried this. I am reading up on it. Anybody with good info on doing this please pass it on. Read my story below.
So I finally upgrade to Mojave on both 2013 Mac Air and 2105 Pro. On the same day boy that was stupid.
Bottom line is that in TM my Cache are missing on Mail dating back to eternity. Both computers and different TM external hard drives.
I go through multiple reinstalls on Mojave. The 2nd reinstall we (Mac Support) wiped the hard drive clean and wiped one of my 2 back up TMs clean. After reinstalling it we then just opened mail. Nothing else was set up, just mail. I went to the mail server and cleaned it out so we only downloaded 10 messages. Anyhow I have a totally clean flash drive (2013 Air) and Time machine is clean. After downloading just those few emails I back them up to the clean TM. And then go back into the Time Machine and sure enough all the messages are there but all the cache is missing on all the messages.
The only good thing about this is I can restore to Mail from TM and the cache will reappear in the "restored mail box folder" in Mac Mail.
But I am guessing on which message is the one I want to restore or I can restore all messages with a giant folder with all caches restored.

So I do a data collection send it to the Advisor.
Today this is the message I receive from the Advisor "I have received an update from the Apple engineers in regards to your case. They have informed me that as of right now they do not have any workarounds or solutions for this issue. They have provided the following statement in regards to this issue:

“Apple Engineering has received reports of this symptom and are investigating. Further updates, if any, may come by way of a software update, knowledge base article or software release.”

In situations like this, they will normally mass e-mail all advisors that have submitted tickets for this issue. I will contact you again if I get anymore communication from the engineers in the future. Going forward, please ensure that you stay up to date on all software updates."

I might give this awhile before moving back to High Sierra but then I will have a lot of data I will either loose or have to find a way to move it from Mojave to High Sierra. I waited until 10.14.4 came out just so I would lessen the chance of a major glitch. So much for that.