Every 1st of the month, command line script to a remote azur sftp services is interrupted

Dear Support,

WinSCP Version 5.17.5 (Compilation 10414) (OS 6.1.7601 Service Pack 1 - Windows Server 2008 R2 Standard)

We have a WinSCP script that regularly, for years and all along the day, uploads/downloads files from/in different FTP/SFTP Servers.

Few months ago, we started a new connection with a Microsoft Azure SFTP Services in SSH.

The script still work during the month, but every 1st of the month, the script stops and asks a manual prompting:
When after that, and for all the remaining days of the month, it works great:
It looks like either the local cache is cleared every 1st of the month (but we identified no parameter that would command so) or that the remote SSH is changed every 1st of the month, but the chain xx:xx:xx:xx:xx:xx:xx:xx:xx:xx:xx:xx:xx:xx:xx:xx yyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyy= remains the same between the 1st day of the month and the other days, so we assume the remote Azure SFTP don't change its key (but is our analysis the right one?)

So at this stage, we don't know how to move forward..
Thanks for your help
Emmanuel

Please post full logs from the 1st day of month and from any of the previous days.
Note that the hostkey is not a secret, it's a public key. Do not worry about posting it publicly.