Possible heap corruption when allocating after running multiple threads about nim HOT 3 OPEN

import md5
import std/[atomics, cpuinfo, monotimes, strformat, strutils, sugar, terminal, times]

const batchSize = 8000

type
  SharedObj = object
    prefix: string
    done: Atomic[bool]
    counter: Atomic[uint32]
    sol: Atomic[uint32]
  Shared = ref SharedObj

proc worker(arg: (ptr SharedObj, int)) {.thread.} =
  let (s, v) = arg
  while not s.done.load(moRelaxed):
    let offset = s.counter.fetchAdd(batchSize, moRelaxed)
    for n in 0'u32 ..< batchSize:
      let i = offset + n
      let h = (s.prefix & $i).toMD5
      if h[0] == 0 and h[1] == 0:
        var val = s.sol.load(moRelaxed)
        while i < val and not s.sol.compareExchangeWeak(val, i, moRelaxed):
          discard
        s.done.store(true, moRelaxed)

proc solve(input: string): uint32 =
  let s = Shared(prefix: input)
  s.sol.store(uint32.high)
  var thr: array[20, Thread[(ptr SharedObj, int)]]
  doAssert countProcessors() < thr.len
  for thr in thr.toOpenArray(0, countProcessors() - 1).mitems:
    createThread(thr, worker, (cast[ptr SharedObj](s), 15))
  joinThreads(thr.toOpenArray(0, countProcessors() - 1))
  s.sol.load

proc part1(input: string): string =
  $input.solve

proc colorizeTime(t: float): string =
  # Crash happens here
  let s = fmt"{t:.3f}"
  if t < 0.5:
    ansiForegroundColorCode(fgGreen) & s & ansiResetCode
  elif t < 1.0:
    ansiForegroundColorCode(fgYellow) & s & ansiResetCode
  else:
    ansiForegroundColorCode(fgRed) & s & ansiResetCode

proc timeit(f: (string) -> string, inp: string): (string, float) =
  let startT = getMonoTime()
  var ans = f(inp)
  let endT = getMonoTime()
  let t = endT - startT
  return (ans, float(t.inMicroseconds) / 1_000_000)

proc run() =
  # Needs the strip call to fail
  let contents = readFile("input.txt").strip
  let (ans, t) = timeit(part1, contents)
  stdout.write fmt"Part 1: "
  echo "$1  Elapsed time $2 seconds".format(align(ans, 54), colorizeTime(t))

run()