Why T-log is not truncated after T-log backup?
The 8th row in your first screenshot shows what appears to be an active VLF in the file. If there is a single active log record in a VLF then the overall VLF will be treated as active log and that VLF cannot be truncated.

If the server is SQL 2016 SP2 or later, you can use the below DMF to get information on the VLFs and check the active state and size. You will likely find that the active VLF is around 350 MB, and if even one byte is active, the entire VLF is considered active and backing up the log won't mark it for truncation.

SELECT s.[name], l.vlf_size_mb, l.vlf_active
FROM sys.databases s
CROSS APPLY sys.dm_db_log_info(s.database_id) l


Further reading:

What are virtual log files in a SQL Server transaction log?

SQL Server Transaction Log Backup, Truncate and Shrink Operations