BUG #16744: ts_headline behaves incorrectly with <-> and proximity operators

The following bug has been logged on the website:

Bug reference:      16744
Logged by:          Stas Obydionnov
Email address:      stas@hellofyllo.com
PostgreSQL version: 12.3
Operating system:   runs on AWS RDS
Description:

When running the following code
    select ts_headline('Alpha Beta Gama', phraseto_tsquery ('alpha gama'))

or 
    select ts_headline('Alpha Beta Gama', to_tsquery ('alpha <-> gama'))
I would expect the result be not to be highlighted, however the result looks
like:
    <b>Alpha</b> Beta <b>Gama</b>

The same behavior is found for the following operator:
    select ts_headline('Alpha Beta Gama Delta', phraseto_tsquery ('alpha <3>
gama'))