Penalties for "stranded" visitors???

BR did extend such but now has begun counting again. If they do it again nationwide I do not know. The US is similar. Many have gone to Mexico and other countries, then quarantined for 15 days and a neg test. (They do not count your arrival date).
I would hope the PF would do a case-by-case extension. (Have any communications and latest government restriction.
US has the STEP program which is a program for citizens traveling abroad. If signed up (free) they keep you notified of travel changes from CDC, CBP, and State Department. Look on the CA site for something as such.

I have been in a very similar situation, and a friend of mine as well, here's how it went: Our flights had been cancelled just like yours, and it wasn't possible to get a flight before expiration of our visas. In my case it was only 7 days, but my friend's flight had been cancelled multiple times and he was 57 days over the 3 months.

We had a Brazilian friend call the Federal Police about it, and they told him not to worry: They said that if someone overstays because of flight cancellations, there will be no fine. However, at the airport, when leaving the country, they said that there's nothing they can do, except giving us the documents to pay the fine (700R$ for me, 5700(!) for my friend). They said that we'll have to appeal the decision by e-mail, in my case I wrote to migracao.rj@dpf.gov.br, because I had been fined at the airport in Rio. I don't remember where I found the address, but I guess you can use it anyway and they will answer with the correct address. Communication was fast and easy, even though I don't speak a lot of Portuguese - Google Translate is my friend .

You will need to send them some kind of proof that your flights have been cancelled, so keep the cancellation e-mails or make screenshots of the cancellation notice.

In my case, they cancelled the fine without further ado, my friend however had waited too long and had to pay it. You have 10 days to appeal the decision, don't miss this deadline.

if you purchased the ticket with your credit card, there may be trip cancellation insurance. wouldn't hurt to ask if they will cover the overstay fees