(report came in thru support, from which i built a generalized test case!)

function init()
  clock.tempo = 90
  div = 1
  start()
end

function start()
  if clock.threads[test_clock] then
    clock.cancel(test_clock)
  end
  test_clock = clock.run(step_through)
end

function stop()
  clock.cancel(test_clock)
end

function step_through()
  while true do
    clock.sync(div)
    print(clock.get_beats())
  end
end

results

note that div = 0.5 is totally stable!

div = 1 ✔

7254.0
7255.0
7256.0
7257.0
7258.0
7259.0
7260.0

div = 0.9 ✔

> start()
27.0
27.9
28.801
29.7005
30.6
31.501
32.4005
33.3
34.201

div = 0.8 ✖️

268.981
268.9825
268.984
268.9855
268.987
268.9885
268.99
268.9915
268.993
268.9945
268.996
268.9975
268.999
269.0
269.6
270.4005
271.201
272.0

with this case, it seems like the clock slips and plays catchup -- once it lands on a whole beat, the resumes continue for a while at the specified div. but, they quickly lose track again.

please lmk if i can provide any additional testing or repro cases!