RTD delay and first RTD does not trigger any event calculate

[ShieldTrade]

xloil 0.18.7
Excel 64

I experience a delay in pyclock ( random from 50 to 150 mls ) and, when the first yield is received in excel, the event calculate is not fired. Following yields normally trigger the event calculate.

sheet1
G10 = pyclock (10)
for cell G11 and G12 change format and use custom format hh.mm:ss.000

sheet 2
C10 = Sheet1!G11+Sheet1!G12+1

Save, close, reopen and press F9.

This is part of a more difficult issue that I am trying to debug where the event calculate triggers a macro ( I use VBA ) which copies the data received in another sheet of the same workbook. It works fine for a random time ( usually from 5 to 24 hrs ) then excel crashes and nothing is printed in the xloil log. The task manager does not show any problem for memory and cpu usage. Before copying I disable events and set calculation to manual. I suppose a thread conflict is the cause but I do not see any in my code. I am deconstructing and testing small part of it.

import xloil, datetime, asyncio
import pandas as pd

@xlo.func
async def pyclock (secs):
    while True:
        data_df = { 'time': [], 'event': [] }
        time = datetime.datetime.now()
        data_df['time'].append(time)
        data_df['event'].append('data received')
        data_df['time'].append(time)
        data_df['event'].append('data received')
        df = pd.DataFrame(data_df)
        df = df.set_index('time')
        yield df
        await asyncio.sleep(secs)`

VBA code in sheet 2
Option Explicit
Private Sub Worksheet_Calculate()
     MsgBox "  calculate sheet 2"
End Sub

[ShieldTrade]

Update

Whenever you change something in python and the reload module is called, the first yield does not trigger the event calculate.

Happy Christmas to @cunnane and to all xloil community!!