P: Auto settings resets back to defaults when you apply auto settings to a large number of photos and then adjusts or apply (via copy settings) an individual adjustment..

• Select a number of photos say about 200 on the film strip.
• auto sync on
• Hit auto to adjust the settings automatically. Can be done on library or develop module does not matter to the issue

Lightroom will adjust the settings automatically and you can go to different photos and the automatic settings would have been applied to all the photos.

• Now adjust one setting on one photo with auto sync on (eg. bring the vibrance or saturation down back to 0 or a different amount….in this case vibrance to -5)

Browse the rest of the photos … 

The rest of the photos should now have auto adjustments applied + the vibrace slider to where you adjusted it manually. However when I do this other than some photos that are in immediate to left and right of the photo that was active while I did the vibrance adjustment the rest of the photos has the auto adjustments back down to 0 and the only vibrace is at -5. This seems to affect tone and vibrance saturation.. not presence (probably because auto does not affect those settings)

The behaviour is the same with the smart preview or the source files plugged in. 

I have noticed this issue affecting some lightroom versions since versions that came since 2020. I have reported this about 4 years back and did not get a satisfactory solution. Only some lightroom versions affect from this issue, as I don’t think I have noticed this for the versions before the latest for an year or so.

Here is the link to that thread from back when I posted about it.
 

Lightroom Classic version: 15.4.1 [ 202606201310-b9f148a4 ]
License: Creative Cloud
Language setting: en-US
Operating system: Mac OS 26
Version: 26.5.2 [25F84]
Application architecture: arm64
Logical processor count: 10
Processor speed: NA
SqLite Version: 3.36.0
Adobe GSDK Version: 1.4.0.213

Power Source: Plugged In, 95%, Charging
Built-in memory: 16,384.0 MB
Dedicated GPU memory used by Lightroom: 992.3MB / 12,124.1MB (8%)
Real memory available to Lightroom: 16,384.0 MB
Real memory used by Lightroom: 832.6 MB (5.0%)
Virtual memory used by Lightroom: 438,766.6 MB
Memory cache size: 447.2MB
Internal Camera Raw version: 18.4 [ 2608 ]
Maximum thread count used by Camera Raw: 5
Camera Raw SIMD optimization: SSE2
Camera Raw virtual memory: 145MB / 8191MB (1%)
Camera Raw real memory: 147MB / 16384MB (0%)

Cache1: 
Final1- RAM:38.0MB, VRAM:184.0MB, FF5BA82E-5AF6-4F81-850A-260513EC1534.dng
Final2- RAM:88.0MB, VRAM:271.0MB, A3360C88-D992-4A1A-AA51-7A4DAA4C38FE.dng
Final3- RAM:37.0MB, VRAM:226.0MB, 1DD746BB-20DD-44A7-910F-13AFD5227795.dng
NT- RAM:163.0MB, VRAM:681.0MB, Combined:844.0MB

Cache2: 
m:447.2MB, n:103.3MB

U-main: 213.0MB

Standard Preview Size: 3024 pixels
Displays: 1) 3024x1964

Graphics Processor Info: 
Metal: Apple M1 Pro
Init State: GPU for Export supported by default
User Preference: Auto
Enable HDR in Library: OFF
GPU for Preview Generation: Off (S5_11)

Installed Plugins: 
1) AdobeStock
2) Aperture/iPhoto Importer Plug-in
3) Flickr
4) Imagen plugin
5) jf Collection Publisher
6) MIDI2LR

Config.lua flags: 

Nikon d810 and D4s