Issues not happening on Firefox or Edge

I assume it's not in a quoted string?  Per spec, commas must be in quoted strings in content-disposition fields.  https://tools.ietf.org/html/rfc6266 (See definition of value.  token is defined in RFC 2616).

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The issue was that the comma was not in a quoted string. 
per spec, it is mentioned in Appendix D (page 13) of https://tools.ietf.org/html/rfc626. thank you mmenke@chromium.org!

It was not easy to figure out that chrome considers a comma a disallowed character. Not at all clear and searching online does not show that info. e.g. https://en.wikipedia.org/wiki/Filename 
Where is that info available (besides for the quite obscure reference in https://tools.ietf.org/html/rfc626)?

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RFCs (And WHATWG "living standard" docs) are the official standards that all browsers are based on, at least in terms of network protocols.  They specify HTTP, DNS, FTP, and other network protocols.  It's actually in the grammar rule (Second 4.1) of https://tools.ietf.org/html/rfc6266.  It's also in the obsolete https://tools.ietf.org/html/rfc2616 (Section 19.5.1), which is the original HTTP/1.1 specification.

Commas unfortunately have special meaning in HTTP headers, and, with a few exceptions, are generally only unquoted when "Foo: a, b" is the same as:

Foo: a
Foo: b

Since, per spec, it's safe for proxies and browsers to merge the latter into the former.

Also worth noting that searching for: content-disposition spec

Will get you a link to the RFC as well as an older version of the spec (Which is, unfortunately, is even older than RFC 2616 and does not include an explicit rule about what values should look like).

The reason you get a somewhat confusing "ERR_RESPONSE_HEADERS_MULTIPLE_CONTENT_DISPOSITION" is the aforementioned comma line-merging behavior.

Anyhow, I'm going to go ahead and WontFix this for the moment.  I think that Chrome's behavior is the more correct one here, and reluctant to be more lax in what we accept here.

[+annevankesteren]:  FYI.  Feel free to weigh in (Or not).

(I agree with your analysis.)