PnP.PowerShell Set-PnPClientSideWebPart, cannot update linkUrl

[gbd35]

I am migrating from SharePoint 2013 to Sharepoint online and I want to transform my wiki pages to modern pages.
I use the command ConvertTo-PnPPage, all is ok.
And now I need to transform all the links "Previous_Mypage.aspx" in "MyPage.aspx" for images webparts of all moderns pages but i cannot update the link associate with the image on the webpart.
I use this code :
$clientPage =Get-PnPClientSidePage -Identity $Page["FileLeafRef"]
$AllWebParts = $clientPage.Controls
foreach ($WebPart in $AllWebParts)
{
$props = $WebPart.PropertiesJson | ConvertFrom-Json
$linkUrlNew = $props.linkUrl -replace $OldString,$NewString
$props.linkUrl = $linkUrlNew
$stringifiedProps = $props | ConvertTo-Json
Set-PnPClientSideWebPart -Page $Page["FileLeafRef"] -Identity $webpart.InstanceId -PropertiesJson $stringifiedProps
}

The properties linkUrl doesn't exist while i can see it in the PropertiesJson when the field Link of the webpart is empty :
[DBG]: PS D:\AppDSI\PPRGLB\Script_SharePointOnline\AuditBrokenLinks> $WebPart.PropertiesJson
{"imageSourceType":2,"captionText":"","altText":"","linkUrl":"","overlayText":"","fileName":"Books-on-a-shelf.jpg","imgWidth":5700,"imgHeight":3802,"alignment":"Center","resizeCoefficient":0.42542372881355933,"resizeDesiredWidth":502,"fixAspectRatio":false}

Anyone has this issue? Any suggestions? Thank you in advance for your help and sorry for my English !