Search boxes not working (+workaround)

HOW TO FIX: Some of the search boxes on the CPRR.org website, unfortunately, have stopped working, falsely reporting "No results found"

This is a search engine bug related to the search being of both the CPRR Museum website (at CPRR.org) and the CPRR discussion group (at discussion.cprr.net).

For example if you search for "railroad" the search box generates the following search, using a comma:
"railroad site:CPRR.org,discussion.cprr.net"
which no longer works.

WORKAROUND:
To correct the search, so that it correctly returns results, all you have to do is to change the search to:
"railroad site:CPRR.org OR site:discussion.cprr.net"
(instead of using a comma and without the quotes) and resubmit the search.

Of course, instead of railroad, use the search term(s) that you actually want to find.

What's really weird is that this new bug affects multiple search engines: DuckDuckGo, Google, and Bing.
Has the syntax for multiple site searches changed? If so, why haven't the search engines updated their handling of site search boxes so that searches will still work?

Can anyone explain what is going on, or have a better workaround???

Can anyone suggest how to modify the webpage code URL that DuckDuckGo generates for the sitesearch box,
https://duckduckgo.com/search.html?site=CPRR.org,discussion.cprr.net&prefill=Search CPRR Museum
so that it generates the correct "OR site:" syntax, instead of the broken "," syntax?