BUG #16862: Unexpected result of checking for null "IS NOT NULL" in function

The following bug has been logged on the website:

Bug reference:      16862
Logged by:          Дмитрий Иванов
Email address:      firstdismay@gmail.com
PostgreSQL version: 12.5
Operating system:   Windows 10
Description:

Good day!
1. Create TABLE: "bpd"."group" AND view
2. Create FUNCTION:
    DECLARE: egroupV "bpd"."vgroup"%ROWTYPE; view
    DECLARE: egroupT "bpd"."group"%ROWTYPE; table
3. Retrieving data checking the result in function:
     SELECT * INTO egroupV FROM "bpd"."vgroup" WHERE id = iid;
     SELECT * INTO egroupT FROM "bpd"."group" WHERE id = iid;
        IF (egroupV IS NOT NULL) THEN => state OK
        IF (egroupT IS NOT NULL) THEN => state OK
        IF NOT(egroupV IS NULL) THEN => state OK
        IF NOT(egroupT IS NULL) THEN => state OK
4. ALTER TABLE "bpd"."group" ADD COLUMN
5. Retrieving data checking the result in function:
     SELECT * INTO egroup FROM "bpd"."vgroup" WHERE id = iid;
     SELECT * INTO egroupT FROM "bpd"."group" WHERE id = iid;
        IF (egroupV IS NOT NULL) THEN => state OK
        IF (egroupT IS NOT NULL) THEN => state NOT WORK???????????
        IF NOT(egroupV IS NULL) THEN => state OK
        IF NOT(egroupT IS NULL) THEN => state OK