Macbook Pro M3 using swap even though there is a lot of free memory available

What does Activity Monitor show for Swap usage? The third party utility you are using may not be using the correct information within macOS especially if it is a Linux based utility. If the system is actually 18GB of Swap, then you have a major issue.

Edit: You can use the following macOS command to retrieve the Swap usage (the value is the one after "used = "):

sysctl  -a  |  grep  -i  'swapusage'



Or with a slight modification:

sysctl  -a  |  grep  -i  'swapusage'  |  awk '{print "Swap = "$7}'

Or if you just want to see that value without any text:

sysctl  -a  |  grep  -i  'swapusage'  |  awk  '{print $7}'

Results in the following output respectively:

vm.swapusage: total = 4096.00M used = 3160.25M free = 935.75M (encrypted)

Swap = 3160.25M

3160.25M

My uneducated guess would be either

A Badly behaving Third Party Application that is not using the 96 GB of Unified Memory coupled with a Memory Cleaner Application that is cleaning the Memory ( RAM )

Resulting in the Operating System being forced to create and use SWAP files

If the second of the two is used

Remove this application and then Test if the SWAP file are being created

I'd try rebooting.

How much RAM do you have?