i can confirm that the behaviors mentioned in the original article are correct
mail will become not findable/missing/deleted/dissapear causing either (1) only headers to show up or (2) when messages are moved in any way they cant be found and probably need to be considered deleted.
any way that mail is moved makes this happen when mail is moved a 2nd time. moving mail by any way: (1) using the Move To icon in the menu bar (2) through Rules or (3) through drag/drop can all cause this.
the only way that Mail is currently useable (in a reliable way) is if incoming email is left on the server and not downloaded to your computer locally.
having said that, my email can seem to handle being downloaded off the server into a local mailbox/folder the first time it is moved/downloaded to a mailbox/folder locally.
but the same mail may (usually?) become lost if its moved to a different (2nd) mailbox/folder at any point later.
in my setup, i have 4 mail accounts, all IMAP, but i choose to always download mail from the server and store it in various local mailboxes/folders on my mac. Catalina Mail will download and apply my rules to get the email into the right local mailbox/folder. it is stable there. it won't be lost.
but when/if you move that mail subsequently, it either will lose its content (showing only headers) or become totally missing (not findable - probably deleted, but certainly not accessible).
since i was able to confirm the above problem, i havent done any more experimenting with it to know if happens all the time, or just some of the time.
additionally, i can also confirm that doing a restore of new mail downloaded into local mailbox/folder that becomes subsequently lost can in fact be restored from Time Machine as i have done that for 1 (one ! critical mail that was lost through xfer to a different folder by Rules). took 20 minutes to get it back, but it does work.
the confusion on this point cited in the macrumors article probably stems from the timing of the Time Machine backups. if there was a Time Machine backup done after the email got onto your system (initially safely),it is of course backed up by Time Machine, and fully recoverable.
there is something wrong in the way that Mail is tracking the email as it gets moved around internally.
as a workaround, for all email that are critical i simply now drag to the desktop all mail i need to have for critical work, to ensure i have a copy (albeit on the desktop...).
in many other (less critical) aspects, Catalina Mail appears very rough and not finished as a product. many controls are missing that simply should be there.
its almost like they took iOS Mail and through the Project Catalyst black box made a mac version that doesnt work...on a mac.