Beyond the fact that security on Telegram is a joke (E2EE not enabled by default, only available in 1-to-1 chats, groups chats are all unencrypted, homespun encryption algo), they have never had a full, independent audit of their encryption standard.
It looks like there are a handful of papers that looked at parts of the earlier standard Telegram used (MTProto 1), but nothing on the current version (MTProto 2).
https://courses.csail.mit.edu/6.857/2017/project/19.pdf
https://eprint.iacr.org/2015/1177.pdf
https://eprint.iacr.org/2015/1177.pdf
Anyway, long story short, Delta Chat has had independent audits several times. I'd say that says it all, really.
https://delta.chat/en/help#security-audits
(Also, thanks for introducing me to Delta Chat, was unaware of the project up to now. Neat stuff.)