An update - like I said initially, this had nothing to do with ddos-protection. Steam master server downtimes crippled the servers just as well, and they were pretty frequent this year, I'm surprised so very few people were aware of this, as some of the downtimes lasted over 2 hours (each time forcing FaceIT and MM to close their queues).
Valve finally acknowledged this a little over a month ago:
Quote:
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– Updated Linux dedicated servers Steam Client layer to the latest version which significantly improves TCP reconnect timeout in case of upstream connection drop.
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https://blog.counter-strike.net/inde...2019/08/25188/
Haven't had the issue since.