SQL Server Instance How to recover memory used
Is this Normal?

Yes.

my boss and colleagues state that this is not normal behaviour for the database

SQL Server will grow its memory usage up to your configured max server memory and only reduce its memory usage if the system runs low on physical memory.
If the system has plenty of available RAM, SQL Server will use it to cache data and execution plans, and to have memory on-hand to run queries.
If Windows sends out a global &quot;low memory&quot; signal SQL Server responds by triming its caches and de-allocating memory.
This is explained in detail in the docs:

The default memory management behavior of the SQL Server Database
Engine is to acquire as much memory as it needs without creating a
memory shortage on the system. The SQL Server Database Engine does
this by using the Memory Notification APIs in Microsoft Windows.
When SQL Server is using memory dynamically, it queries the system
periodically to determine the amount of free memory. Maintaining this
free memory prevents the operating system (OS) from paging. If less
memory is free, SQL Server releases memory to the OS. If more memory
is free, SQL Server may allocate more memory. SQL Server adds memory
only when its workload requires more memory; a server at rest does not
increase the size of its virtual address space.
Max server memory controls the SQL Server memory allocation, compile
memory, all caches (including the buffer pool), query execution memory
grants, lock manager memory, and CLR1 memory (essentially any memory
clerk found in sys.dm_os_memory_clerks).

Memory Management Architecture Guide - Dynamic Memory Management

Is there a way to release memory without rebooting the instance?

You can reduce the max server memory and SQL Server will trim its caches and reduce its memory usage online.  After it drops, you can increase max server memory again and SQL Server will re-grow the caches over time.
But this procedure is operationally pointless.  The memory isn't needed for anything else, and if it were, SQL Server would respond to the system low memory signal by reducing its memory utilization.