You've already said you only use Defender, so that leaves, as I also indicated in my first post on this I think, the possibility of any other programs interfering.
My post also suggested other possible reasons the status of the service might be changed- though I've not (yet) seen anyone else say this had happened.
There is this thread:
A complete Windows index on my Win 11 24H2 machine has around 500'000 items. Every now and then I notice that search becomes extremely hit and miss. When I look at the indexing status, I notice that Windows has restarted indexing from scratch. After indexing is complete (which can take several...
www.elevenforum.com
- where the user finds a workaround in post #9.
Sadly too many respondents in Windows search threads just recommend 3rd party search tools, and don't address the question asked.
I use both- the right tool for the job at the right time- so Windows search only indexes my personal files, whilst the 3rd party too indexes all files including system files.
Disk is good.. seems you only have the one disk.
Indexing.. the most basic distinction is to index ALL files - including O/S files- or only personal files in certain locations.
This choice is a single click in Settings.
To set the classic search setting in Windows 11, go to
Settings > Privacy & security > Searching Windows and select
Classic under "Find my files". This mode indexes only your desktop, documents, pictures, and music folders, reducing system resource usage. You can customize included folders via the "Customize search locations"
There you should also see the search status: how many files & folders indexed, and the status of indexing.
If this shows e.g. Indexing is temporarily paused or Speed is reduced because of user activity etc... Look at your task manager. Is there significant resource usage?