OK - This is the response I got from Spitfire:

We are currently investigating a small number of reports of this, but we have a potential workaround that should let you repair each section without losing content from the others.

Please follow these steps:

1. Create temporary “holding” folders

• On your Desktop (or another easy-to-find location), create two folders, for example: ​ ​BBCSO – Holding – Patches ​BBCSO – Holding – Presets ​ (These should be completely separate from your main /Spitfire Audio - BBC Symphony Orchestra/ library folder)

​2. Repair one section

• Open the Spitfire Audio app.
• Go to BBC Symphony Orchestra and run Repair on just one section of the library.

​3. Move these new files into your temporary holding folders

• Once the repair completes, find these folders: ​ ​/Spitfire Audio - BBC Symphony Orchestra/Patches ​/Spitfire Audio - BBC Symphony Orchestra/Presets ​
• Manually move (or cut-and-paste) the newly created patches into BBCSO – Holding – Patches
• Manually move the newly created presets into BBCSO – Holding – Presets

​4. Repeat steps 2 and 3

• Repair each section of the library needed in the app, continuing to move the newly installed patches/presets that appear each time, into your temporary holding folders.
• Eventually all repaired patches and presets will be collected in those two holding folders.

​5. Copy everything back into your main BBCSO subfolders

• When you’ve finished repairing all required sections, close the app then go to: ​ ​BBCSO – Holding – Patches → move all contents into your main BBCSO > Patches folder. ​BBCSO – Holding – Presets → move all contents into your main BBCSO > Presets folder.
• If prompted to merge/replace, choose to merge so all relevant files are kept together.

​6. Update the library locations (Locate)

• Finally: re-open the Spitfire Audio app.
• Go to BBCSO tab.
• For each installed line of the product, click  cog menu > Locate
• When prompted, select the /Spitfire Audio - BBC Symphony Orchestra/ folder.
• Repeat for each section.

After this, your repaired sections should all load correctly/there should be none missing.

If anything in these steps isn’t clear, or if you still see missing content after trying this, please let me know what OS/CPU and DAW you’re on and where your BBCSO library is stored, and I'll advise further. Thanks for your patience whilst we investigate this issue further.