I guess, I catched the bug with naming of js translation files in sites/default/files/languages/*.js in my case I see base64_hash_0.js, base64_hash_1.js etc.
Perhaps when a few users open the page after cleaning page, they both try to generate the same file for js translations.
Use FileSystemInterface::EXISTS_REPLACE in _locale_rebuild_js for rewriting file on creating.
No to create new with numbers in the end provided by FileSystemInterface::EXISTS_RENAME.
Needs review
9.5
Next step would be to add a test case showing the issue.
Would be good to get a full list of steps to reproduce this.