Thx for the data Jamal - I think this is a bug based upon my testing.
1. I followed your workflow in a new project and confirmed the sort ascending/sort descending was grayed out - AGP used the "new" Join cardinality of 1:M by default - it seems there's no way to do a traditional 1:1 if the software detects 1:M.
2. Tested in ArcMap where supported cardinalities for a Join are 1:1 and M:1. Sort ascending/sort descending worked.
3. Back in ArcGIS Pro, created a copy of T1 called T2 and removed duplicate records. Create a traditional 1:1 join and sort ascending/sort descending worked.
4. Ran the Join Field GP tool on copies of L1 and T1 - tool modified existing L1 data and used 1:1 cardinality I believe. Sort ascending/sort descending worked.
My belief is that the 1:M cardinality for a Join in ArcGIS Pro 2.7.2 (maybe earlier) is the culprit with the sort ascending/sort descending not working. I recommend you contact Esri Support Services and log this as a BUG.
Let me know if I can provide further assistance. Thx!