The subjunctive is called for because of the superlative l'un des plus rudes. In French, a statement in the superlative is often accompanied by the subjunctive because claiming that something is the highest, strongest, harshest, etc. is always to be taken with a bit of doubt. Only when one wants to express absolute certainty, the indicative is used.

https://www.lawlessfrench.com/grammar/superlative-subjunctive/

Hi G,

According to Laura Lawless's "subjunctivisor", se souvenir que does not take the subjunctive at all.

So perhaps there is an error that needs to be flagged up?

Jim

That's one of the grammar rules I've never come across before. Makes perfect sense though.