Proxy / Explicit VS automatic / Difference in usage of port 4070

[cnotin]

Plan

Premium

Country

France

Operating System

Windows 10

Desktop client

Latest version 1.1.30.658.gf13cde74

My Question or Issue

I've noticed a different behavior in Spotify Desktop client when using "Automatic detection" or "HTTP" proxy settings.

Context: I'm behind a proxy automatically configured via PAC file in Windows.

When using "Automatic detection" in Spotify proxy settings, the client tries to connect to Spotify access points on port 443 through the proxy (ex. proxy request: CONNECT gew1-accesspoint-b-wl7f.ap.spotify.com:443 HTTP/1.1). However, this is not valid HTTPS/TLS connection so the proxy blocks and streaming doesn't work.

When using "HTTP" in Spotify proxy settings (with explicit IP of the same proxy as used previously), the client tries to connect to Spotify access points on port 4070 through the defined proxy (ex. proxy request: CONNECT gew1-accesspoint-b-7q0z.ap.spotify.com:4070 HTTP/1.1). This is allowed by our proxy and streaming works supposedly because the proxy doesn't try to interpret this as TLS.

My question is: why this difference in behavior between two cases? Shouldn't it be the same?

I'd prefer if Spotify tries port 4070 when "Automatic detection" is selected, or actually that it tries both.

Note that contrary to other posts, I'm not asking to stop using the 4070 port, actually it's the opposite I'd prefer the client to use it even through the proxy as it seems to not be blocked.

Please let me know if you need more info.

Thanks!