Andrea,
I downloaded GeoServer 2.26-SNAPSHOT locally on my machine, and followed the same procedure as in production, installing the 2 plugins: first, only COG-S3, but it does not appear in the “Store” menu, then installing S3 Geotiff, and this, as it had already happened in production, worked.
Unfortunately, GeoServer does not complain about anything in the log, it is as if the plugin had not been installed.
Best regards,
Fernando Quadro
https://fernandoquadro.com.br