Apple occasionally break a few things with their operating system updates and it has come to pass that the latest version of OSX: Big Sur 10.3 does this for IP over thunderbolt.
If your fast connection between your Mac and your QNAP NAS has suddenly stopped working then this is why.
I run IP thunderbolt to my QNAP NAS as this gives me 900mb/sec connectivity for reading and writing files (far faster than mechanical drives or sata based SSD). Looks like I will need to run this at 100mb/sec via gigabit ethernet till they sort the issue. If you have the same problem then you can downgrade using time machine or wait for the fix.
The vast majority of the work I do is Windows PC based and my 10GB connectivity still works fine to deliver the 900mb/sec.
Update: 03.05.2021 19:58
Big Sur 10.3.1 is a security update and does not fix the issue with IP over thunderbolt. Lets see what 10.3.2 can do when ever it comes out.
Update: 17.05.2021
I have joined the Mac OS beta program and downloaded the third Beta of 10.4, this has fixed the Thunderbolt issues when connecting to my QNAP. Not generally a good idea to run Beta OS but was advised this was stable and it does seem to work fine.