some sql_trace related issue on 7.3.4

Server 7.3.4.0.0. Client SQLPlus (NT) 3.3.4.0.0.

I have done "ALTER SESSION SET SQL_TRACE=TRUE;' many times to produce trace file for
tkprof.

In the middle of day, for whatever reason. after issue this statement,  a trace file with 0 byte was created in 'user_dump_dest' directory.  background info: this problem was happened shortly  after a 35MB trc file was written and truncated to 'user_dump_dest' without reach to the max_dump_file_size 102400 * 512 = 51MB )

Anybody encountered such a case while using sql_trace ? could it be possible that run out of disk/directory quota ?
Thanks in advance for any hint.

BTW, I also notice that by querying v$parameter NAME TYPE VALUE ISDEFAULT ISSES ISSYS_MOD ISMODIFIED ISADJ DESCRIPTION
---------- --------- ---------- --------- -----

• ---------- ----- --------------
  ---
  sql_trace 1 FALSE TRUE FALSE FALSE FALSE FALSE enable SQL trace

even after 'alter session set sql_trace=true'. Why?

In my opinion, it should read like:

VALUE                    : TRUE
ISDEFAULT             : FALSE

ISMODIFIED             : MODIFIED
ISADJUSTED            : FALSE