Issue is solved. The context for anyone interested from one of Scaleway Engineers [1]:

To give some context, there are a lot of possible path between Cloudflare and Scaleway : France IX in Paris, NL IX in Paris and in Amsterdam, AMS IX in Amsterdam, and the transit in both region (Cogent, Lumen, Arelion). Cloudflare is announcing some prefixes only on some of these links.

We were trying to find the root cause since Monday morning, but as we did not have any errors visible on our side, it was pretty hard to identify the faulty path.

Another fun fact from Scaleway CTO is that Cloudflare started answering ... just ~30 minutes ago. [2]

[1] https://scaleway-community.slack.com/archives/C7Z76CCUE/p167...

[2] https://scaleway-community.slack.com/archives/C7Z76CCUE/p167...