How many Thread routers (not Thread border routers) do you have - AC powered Thread devices like the Eve Energy plug or Nanoleaf Essentials bulbs/strips?
The last time someone had these issues, I recommended to place a thread router near the Nuki lock and that solved the problem for good!
I have at least four thread routers in every room (easy when every light is a Nanoleaf device). Even only one device per room would create a rock solid Thread mesh network.
You know, your lock should have one or two alternative paths back to your Thread border router (SmartThings hub). It’s all radio signals after all…