Water hammer - where to place arrestor
If it is only the mixer tap that causes the issue then fit it close to that.

It is not surprising as those short action lever taps shut off the flow quicker than the taps requiring a turn or two to stop the flow.

If two taps cause this then either you need two arrestors or one where their supply pipes combine. The only issue with this is that any length of pipe between the tap and the arrestor is subject to the pressure pulse that causes water hammer and, if it is weak, it could fail.

Had a situation where a large valve (6" pipe valve) could cause water hammer if it was shut too rapidly (supply to a water turbine) but the surge protection could not be close to that valve - it had to be 60 yards before the valve... So that pipe had to be stronger just in case.  The surge protection was a vertical pipe of 32 feet, which would provide a big shower if the valve was shut too rapidly.