RFC Call - Parameter value truncated

Edited, with duplicable code.

I have two RFC function modules:

FUNCTION Z_MATT_TEST1.
*"----------------------------------------------------------------------
*"*"Local Interface:
*"----------------------------------------------------------------------

ENDFUNCTION.

and

FUNCTION Z_MATT_TEST2.
*"----------------------------------------------------------------------
*"*"Local Interface:
*"  IMPORTING
*"     VALUE(I_REASON) TYPE  STRING
*"----------------------------------------------------------------------


ENDFUNCTION.

REPORT ymabitest2.


CALL FUNCTION 'Z_MATT_TEST1'
  DESTINATION 'OTHER_SYSTEM'.

CALL FUNCTION 'Z_MATT_TEST2'
  DESTINATION 'OTHER_SYSTEM'
  EXPORTING
    i_reason = 'The reason'.

These FM exist in OTHER_SYSTEM but Z_MATT_TEST2 has an endless loop so I can debug

  DATA x.
  WHILE x IS INITIAL.
  ENDWHILE.

When I run ymabitest2, in debug on OTHER_SYSTEM, I can see that i_reason contains 'T'.

If I comment out the call to Z_MATT_TEST1, in debug on OTHER_SYSTEM , I can see that i_reason contains 'The reason'.

This only happens if

a) Both FM are called

b) i_reason is supplied with a type C variable or literal. If it is supplied with a string literal or variable, it's fine.