Can't upload R2 incremental cache - recurring 503 Service Unavailable #12413

What versions & operating system are you using?

We have a project, when with wrangler 4.42.2 and @opennextjs/cloudflare version: 1.11.0 we're able to bulk upload R2 incremental cache containing over 17k assets on regular basis. The logs looks as follows:

Populating R2 incremental cache...
Populating remote R2 incremental cache using batch upload...
Using batch upload with R2 credentials from environment variables
Successfully uploaded 17126 assets to R2 using batch upload

Now we would like to update to newest Wrangler and OpenNext, but it seems OpenNext changed the way of bulk uploading R2 cache, which is again error prone. In our logs we can see:

Resource location: remote 
Starting bulk upload of 16974 objects to bucket <bucket> using a concurrency of 50
Uploaded 0% (10 out of 16,974)
Uploaded 0% (20 out of 16,974)

and in the end:

Uploaded 21% (3,720 out of 16,974)
Uploaded 21% (3,730 out of 16,974)
▲ [WARNING] 🚧 `wrangler r2 bulk put` is an experimental command. Please report any issues to https://github.com/cloudflare/workers-sdk/issues/new/choose
▲ [WARNING] Processing wrangler.jsonc configuration:
    - Unexpected fields found in top-level field: ""
✘ [ERROR] Error uploading "<path>"
  Error: Failed to fetch <url>.fetch - 503: Service Unavailable);

In the end it is impossible to upload R2 incremental cache and in the end our deployment fails.

Please provide a link to a minimal reproduction

Any Next.js project with huge number of ISRed pages

Describe the Bug

Regression on R2 bulk update

Please provide any relevant error logs

Uploaded 21% (3,720 out of 16,974)
Uploaded 21% (3,730 out of 16,974)
▲ [WARNING] 🚧 wrangler r2 bulk put is an experimental command. Please report any issues to https://github.com/cloudflare/workers-sdk/issues/new/choose
▲ [WARNING] Processing wrangler.jsonc configuration:
- Unexpected fields found in top-level field: ""
✘ [ERROR] Error uploading ""
Error: Failed to fetch .fetch - 503: Service Unavailable);