Errors on multi-variable assignments of types

Bug Report

MyPy seems to have issues when multi-variable assignments of types to their aliases are involved.

To Reproduce
Run mypy on the following two sources:

from typing import Any

typ_a = Any
typ_b = Any
def test1(x: typ_a, y: typ_b): pass

from typing import Any

typ_a, typ_b = Any, Any
def test2(x: typ_a, y: typ_b): pass

typ_a = typ_b = Any
def test3(x: typ_a, y: typ_b): pass

Expected Behavior
MyPy does not show any errors for any of the function's signatures, since they are all essentially the same.

Actual Behavior
MyPy shows the following errors for test2 and test3:

repro2.py:4: error: Variable "repro2.typ_a" is not valid as a type
repro2.py:4: note: See https://mypy.readthedocs.io/en/latest/common_issues.html#variables-vs-type-aliases
repro2.py:4: error: Variable "repro2.typ_b" is not valid as a type
repro2.py:7: error: Variable "repro2.typ_a" is not valid as a type
repro2.py:7: note: See https://mypy.readthedocs.io/en/latest/common_issues.html#variables-vs-type-aliases
repro2.py:7: error: Variable "repro2.typ_b" is not valid as a type
Found 4 errors in 1 file (checked 1 source file)

Confusingly, if one merges the two source files, no errors are shown.

Your Environment

• Mypy version used: 0.800
• Mypy command-line flags:
• Mypy configuration options from mypy.ini (and other config files):
• Python version used: 3.9.1
• Operating system and version: Ubuntu 20.04

Start by running mypy on the three reproductions in repro2.py and compare how single, tuple, and chained assignments are recorded as type aliases. Trace the type-checking path for those assignments and function annotations; done means all three signatures produce no errors while existing alias and variable checks remain unchanged.