I have a database 40GB, a table Name is T1 with 70 columns and 1000000 records, I run isql command:
connect database 'localhost:/db/db.fdb' user 'sysdba' password 'masterkey';
update set T1 set Column1=0;
the 2nd Statement run with IBExpert is ok, waste time 2 minute. In this case, after 2 minute(and more time), isql can't get back to SQL> prompt.
I change 1st Statement to "connect database '/db/db.fdb' user 'sysdba' password 'masterkey';" everything is ok.

I test the db without automatic garbage collection, this problem same:

gfix -sweep /db/db.fdb

gfix -h 0 /db/db.fdb
isql
SQL>connect database 'localhost:/db/db.fdb' user 'sysdba' password 'masterkey';
SQL>update set T1 set Column1=0;
then can't back to "SQL>"