Immutable distros works great when you want them exactly as they come. Anything else is a shit show IMHO.
As an example, multiple fedora based immutable distros dont have the codecs required to play YouTube videos. You have to either replace the rpm-ostree based Firefox with the flatpaks one or layer ffmpeg over the base system. Both solutions I wouldn’t expect someone without Linux knowledge to be able to do.
First thing I’d do is to look at the client (fedora) journal for anything funky happening.
‘sudo systemctl status nfs-client’
Since it’s random I assume you won’t have any timeout in your /etc/fstab but it might be worth taking a look anyway.
Be aware that if the network drops the NFS will be disconnected and won’t auto-reconnect so this could also be the issue.
I don’t know if it plays well with container mounted volume, but looking at autofs could be a solution to auto-remount the share. I use it profusely for network mounted home directories.