You’ll find below the 2 checkpoints to verify so that your email signature displays correctly when composing:
1 - Go to the folder File/Library/Mail/V10/MailData and delete the folders: RemoteContentURLCache and Signatures
2- In the Mac system preferences, go to Aple Account > iCloud > Dirve > Apps syncing to iCloud Drive and uncheck mail
Feel free to follow these steps with help from your IT team. 
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