With some research, I think I found what was causing it (looks like this isn't a bug).
Telegram Client has a class to determine your phone class (low/mid/high end). Apparently, blurred headers works only for high-end phones.
See below what characterizes phone quality: - Low end phones (PERFORMANCE_CLASS_LOW, true if it satisfies at least one of these conditions): -> Android version < 21 -> CPU cores count <= 2 -> Memory class (system recommended RAM for each app): <= 100mb -> CPU cores count <= 4 AND Average CPU frequency <= 1250 -> CPU cores count <= 4 AND Average CPU frequency <= 1600 AND Memory class <= 128 AND Android version <= 21 -> CPU cores count <= 4 AND Average CPU frequency <= 1300 AND Memory class <= 128 AND Android version <= 24
- Mid end phones (PERFORMANCE_CLASS_AVERAGE, true if it satisfies at least one of these conditions): -> CPU cores count < 8 -> Memory class <= 160 -> Average CPU frequency <= 2050 -> CPU count = 8 AND Android version <= 23
- High end phones (PERFORMACE_CLASS_HIGH): -> None of the previous conditions are satisfied.
This also affects other things like animations, particle and even your in-app camera quality.
As far as I know, there is no option to force enable this feature. The way is to wait if the requirements for the blurred header will be reduced in the future or if there will be an option to force it (maybe even in an unofficial client).