I've been going through and doing the Joomla update to 5.1.0, and have noticed that Akeeba Backup and Admin Tools on many of my sites (I manage 90+) have lost their Download ID. When I look at the Joomla dashboard it shows all DL keys are entered correctly, but if I click on that, neither AB nor AAT are listed in that section. Rebuild doesn't recover them. I have to go to Component | AB/AAT, expand the "Your download ID is missing" then click the link (links are the same color as the text which is hard to use) to go to the DL ID input screen to enter it.
About half of my sites are like this and it's not something I've missed. These 2 extensions are always the first I install, and after installing go to the Enter DL Keys section of Joomla's dashboard. So something is stripping them.