This is something that comes up quite regularly across the Logic forums, so it's not an "11.1" thing, but a more general Logic thing where in some cases, or on some systems, it just seems to be wonky and not working as intended.
I'm not sure I've seen a general solution for all cases (I'll go and have a dig around in case I'm forgetting something), but usually iirc it's permissions or folder-structure related.
Are you having this problem with all plugins (and including Logic's own), or is it only for specific plugins?
If the latter, you may need to create it's own plugin settings folder manually or check the permissions on it, as per this solution:
https://www.logicprohelp.com/forums/...Comment-824280
It's worth checking the permissions on those folders and checking your ~Library/Audio/Presets folder is there as expected. Sometimes pointing Logic to the correct plugin folder by saving a preset there from the plugin concerned is enough to update the path and have it work from then on - you should then see save as default available again.
More details on changing permissions for a misbehaving plugin preset folder:
https://www.logicprohelp.com/forums/...party-plugins/