Disappearing detail when in Drive mode


I am designing a model railroad whereby I am using a room and baseboard previously downloaded from the DLS as a starting point. The design is far from completed. However, as I flip between Survey mode and Drive mode, stuff disappears……track, structures, vegetation, entire areas, etc. When I flip back to Survey mode more stuff appears again, but not all stuff. This only happens as I start to add more and more assets and detail. I am starting to think this is happening only when I use a room template I have downloaded from the DLS as a recent design I started from scratch does not have this problem. But I am not sure.

My iMac hardware/software is relatively old but it does run TS 2019. So while it could be a contributing factor I doubt it, considering the size of the route is really small when compared to routes mainly many miles long.

Can anybody identify what might be going on?

Could the original file be corrupt? Can it be purged or fixed?

Can I export the file and then reimport as a different name?

Are there some settings I should be adjusting (nothing looks out of the ordinary and I have turned off shadows)?

This is very frustrating and I am afraid I will loose a lot of design work.

Any ideas as to how to solve this? Thanks in advance.
I'm no expert, but it sounds like you might be saving changes to a session rather than saving them to a route.
Do the missing items re-appear when you load a session?