Firewalls are positive enforcement, meaning they will only do explicitly what you tell them to.
If you don't have a rule to decrypt telegram, it won't happen.
If telegram is being picked up as a different app, you can create or modify existing App-IDs to get granular enough to exclude/include.
If you are a user on a corporate network that IS decrypting telegram, this isn't the place to talk about offensive sec and product bypassing 🙂
SSL Decrypt is controlled via URL category and you can also use ports within the Decrypt policy, frankly I am shocked that Telegram does not use a pinned-cert, if it did it would be in the Decryption exclusion list Device > SSL Decryption Exclusion however the best thing to do would be (in the absence of any useful info on the Telegram website) carry out a packet capture on the firewall and identify the traffic in question and more accurately check where the app is going URL wise.
A good starting point would be a custom URL category with Telegram added and then a do not decrypt policy above your broad decryption policy, however the packet capture would show the detail and this could also be added to the URL category.
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