In the 'Options' panel of INDI client, there is 'Polling' - Period (ms)
The default value is 1000.
On my understanding this means every 1000ms the driver does query the position of the focuser.
After I set this to 50000 and save the configuration, the value changed but from the log I can see the driver kept querying position every 1 sec.
Did I misunderstand or this is a bug?

Yes it's a bug in the INDI Stable build and is already fixed in GIT. So it will be in the next release of INDI Stable, or you can compile from GIT.

thank you Jasem for your quickly reply.
When will you plan to release the new stable?
I want this feature because there is a Robofocus clone here. I can nearly make it work.
Only one issue is, there is a TTY timeout error after it moved (no issue of the moving). So the position is not refreshed. And the error always shows with "Bogus position:(00 00 00 00 00 00 00 00 00) - Bytes read: -1
I guess this device does not response any query command when it is moving. Maybe to increase the query period could help.

And also, I think the logic of querying position could be bug. It seems to jump into a dead loop after it failed a TTY connection.
It reads out -1 bytes every 1 sec (to the end of the world). This means some resources are not released and it does not do a clean reconnection. And there is no global timeout.
It should release the port and sleep for several seconds to reconnect to the port after a failure.
I'm sure it's a logic dead loop but not a hardware deadlock, because the device can still move after this 'timeout. Moving always works but position readout is dead.

You can use the INDI Nightly build PPA or compile from GIT. If you can find exactly what the problem is, let me know.