BUG #17269: Why is virtual memory usage of PostgreSQL growing constantly?

The following bug has been logged on the website:

Bug reference:      17269
Logged by:          Yu Kikuchi
Email address:      y.kikuchi0714@gmail.com
PostgreSQL version: 9.6.17
Operating system:   Windows Server 2016
Description:

Hi, our team uses the PostgreSQL 9.6 database.
A few days ago, we changed memory size because the ratio of memory was about
70-80%.
Accordingly, we changed sharred_buffers in postgresql.conf.

Before
・memory:64GB
・shared_buffers:16GB

After
・memory:128GB
・shared_buffers:32GB

The radio of memory is about 20% after we changed memory size, but
PostgreSQL uses virtual memory constantly.
PostgreSQL finally ran out of virtual memory a few hours later. 
Why did PostgreSQL use virtual memory although there is an available memory?