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(at)gmail(dot)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