After Windows update 1903 Sticky Notes opens in different windows

As you mentioned, installing the old app may be the best solution;

1. Open Elevated Powershell [windows key + X, Windows Powershell (Admin)].

2. Remove existing app

   Get-AppxPackage Microsoft.MicrosoftStickyNotes | Remove-AppxPackage
   

3. Install old app (this is from my computer, if does not work review own app folder in second section),

   Add-AppxPackage -register "C:\Program Files\WindowsApps\Microsoft.MicrosoftStickyNotes_3.1.54.0_x64__8wekyb3d8bbwe\AppxManifest.xml" -DisableDevelopmentMode
   

4. From memory you will also need to disable apps from updating, this can be done in STORE (maybe pro only) or turn off app provisioning;

   Get-AppxProvisionedPackage -Online | Where-Object {$_.PackageName -like "*StickyNotes*"} | Remove-AppxProvisionedPackage -Online
   

Source One, Source Two

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To review old apps:

1. Take ownership of C:\Program Files\WindowsApps tutorial here.

2. Replace Microsoft.MicrosoftStickyNotes_3.1.54.0_x64__8wekyb3d8bbwe of step 3 above with desired app version.

   Add-AppxPackage -register "C:\Program Files\WindowsApps\APP-NAME-HERE\AppxManifest.xml" -DisableDevelopmentMode