Can't manually update Update_core

Such a weird problem, wondering if anyone else has run into this. I have a site that works fine except I can’t update core automatically. I looked at the update_core.php file & my ftp program was only showing it to be about 4-5K in size, rather than the 29-30K it should be.

When I download the file, sure enough, the file isn’t complete. Everything after line 70 or so is missing.

I’ve tried –
– renaming a good copy of the file & uploading it
– removing the entire wp-admin directory & re-uploading it
– I’ve turned off the 3 plugins this site is using, so tried both uploads with plugins on & plugins off
– I switched my theme to 2016 & tried to upload

My FTP software says it’s uploading the full 29K, but that full content never seems to make the server.

I’m on Rackspace for hosting. This is a multi-site installation, and I was able to update another munched file (image-edit) for this same site.

Any ideas of what else to check or try?

Thanks all!