Last week I raised a couple of tickets with the brid.gy dev, one of which related to truncation length for fediverse using the mastodon api but with larger character allowances than the default 500.
Ticket here: https://github.com/snarfed/bridgy/issues/953
In my case its for pleroma which has an additional max_toot_chars field and on my instance this is set to the default of 5000 characters.. I think perhaps misskey uses this field too..
Anyway, this post serves as a test of the implementation sitting as it does at 541 characters.