This is a very old problem that has been around for years, on and off.
The vmnat process goes to 100% CPU, for seemingly no reason.
I have seen this come and go over many versions of Fusion. Until recently I was using an Intel MBP and the problem would occur about once a month. The only way to fix it is to shutdown all guests and Fusion itself and then restart everything.
I've recently moved to an M4 MBP and installed Fusion 13.6.2
The problem is back and it's occurred several times today already.
I cannot believe that after all this time, this problem is still plaguing the software and has not been fixed.
This bug really needs to be fixed.