Am I taking crazy pills?  Why does this do it? It shows correctly when created, then on save and re-open to find it has changed.
 
I don't think I've had this happen before.

Do you have any actual audio files  in the project? 
Otherwise I think Sonar will just run at whatever your interface is currently set to. 

I did a test where I started a new project at 96/24. Recorded a few tracks. Saved it, and re-opened it ...and it was 96/16.  The FW-1884 can handle 24 bits, and there is no setting for anything but 96kHz in the Tascam interface. Bit rate is controlled in Sonar as far as I can tell.
 
I'm up to date with every driver and program. Just seems strange I can start a project at 96/24, but it feels the need to go to 96/16.

Experimented further, and 48/24 sticks. 48/24 is what I was using before, but I wanted to recorded at the highest possible rates.
 
 

Could a plugin downgrade the bits?

Wild idea - there are usually two settings for bitdepth - one for storage of clip content and one for interface as such.
 
Any conflict?
 
Still recent years I had a couple of daws that had storage set to 16-bit by default - I don't know why and who really use that still.
 
Another wild idea - many interfaces changes number of accessible ins/outs at 96k - and initializing may cause some reset or conflict when hardware settings don't match.

I recorded a few clean guitars and was able to save and reopen as 96/24. Then I put GR5 on there and it stayed correct. I tried it with Amplitube 4 first and maybe that hosed it. I will check when I get back in front of the machine.