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My Wallet Backups folder hasn't been adding any new backups since the end of March (since 3.5.8.7). Maybe there's a setting I need to manually re-check or something. Windows 10 using version 3.5.8.8

You could add a bug report ?

I have no windows, but on my linux I see backups in walletbackups folder for each day

Just in case, you can also backup manually.

!backup
Make sure you have a backup of 1. your wallet.dat and 2. gridcoinresearch.conf files, and 3. the directory: walletbackups. (Without both (1.+2.) you've LOST your Gridcoins.)
They are located in Windows: %appdata%\GridcoinResearch (and for linux: /home/your_user/.GridcoinResearch/ )
Want to install the backup'd files somewhere else?

Thanks. I back up all of these things regularly, so I'm not in jeopardy of losing the coins. I opened a new issue on github: https://github.com/gridcoin/Gridcoin-Research/issues/292

your avatar is the best.