Comment 1 for bug 1818121

I was following https://help.ubuntu.com/community/SettingUpNFSHowTo in two KVM machines using 18.10
and then https://wiki.ubuntuusers.de/Autofs/
(note you need linux-generic to get autofs4 module)

In exports I have
/export/users/testmount 192.168.122.0/24(rw,insecure,no_subtree_check,async)
$ sudo mkdir -p /export/users/testmount
$ sudo touch /export/users/testmount/testfile

After that in my case I can mount it like:
 sudo mount -v -t nfs 192.168.122.55:/export/users/testmount /mnt

Now trying that via automount:
$ echo "/nfs/ /etc/auto.nfs" | sudo tee -a /etc/auto.master
$ sudo mkdir -p /nfs/testmount
$ echo "testmount -fstype=nfs,rw,retry=0 192.168.122.55:/export/users/testmount" | sudo tee -a /etc/auto.nfs
$ ll /nfs/testmount/testfile
-rw-r--r-- 1 root root 0 Mar 1 10:43 /nfs/testmount/testfile

The mount works just fine and it is v4 (from mount)
192.168.122.55:/export/users/testmount on /nfs/testmount type nfs4 (rw,relatime,vers=4.2,rsize=65536,wsize=65536,namlen=255,hard,proto=tcp,timeo=600,retrans=2,sec=sys,clientaddr=192.168.122.226,local_lock=none,addr=192.168.122.55)

It must be something to your local configuration that is different.
I have to ask you to try setting up two clean VMs as well and then modifying the config I used to step by step to match yours.
That should either reveal which change broke your case.
Or you'll end up with a step by step howto that developers can follow to further debug your case.