Why not on est resté
or nous sommes restés
« On » is singular, grammatically speaking, like the impersonal/formal « one » in English, but in the example it appears to be being treated as plural « nous »
I would have thought On est restés was incorrect.
Le, when 'on' is used in place of 'nous', the verb conjugation remains in 3rd party singular ('on est...', 'on a ...', etc) but the agreement rules for adjectives, and for the past participle in compound tenses can be applied.
That is, there can be agreement of the past participle/adjectives with the plural, and gender if applicable, of 'nous'.
However, when 'on' does not replace 'nous', past participle and adjectives remain invariable, without 'agreement'.
Note that the 'need' for agreement is not universally agreed by French grammarians, or universally known even amongst native French speakers - see link to Laura Lawless site.
Kwiziq however routinely uses agreement when 'on' replaces 'nous'.
On can mean either we/one/people (French Subject Pronouns)
https://www.lawlessfrench.com/grammar/subject-pronoun-on/
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