SEPA payments also support chargebacks. Gyms are notorious for making it easy to sign up and hard to cancel, so the other explanation is that people are upset with some of your gyms and disputing the subscription renewals.
Stripe customer support is beyond useless, so I totally get your frustration, but I would encourage you to get to the bottom of this and verify that that your "insufficient funds" theory is correct by calling the customers with the failed payments.
Hey, @gizmo thank you for your reply, I indeed checked. As I said the account affected here is a local business, they make like 1.1K MRR from the same people forever. I've checked personally the disputes and they are all due to these "insufficient funds", also one issue that happened like 3 weeks ago on the same account was that Stripe was incorrectly crafting the SEPA Mandate and for the reason the bank rejected it.
It seems like their own mistake has caused their automatic fraud detection algorithm to incorrectly flag this account. This is speculation obviously, but with this timing...
Are these insufficient funds on long-standing accounts, or relatively new accounts? It seems like it would be pretty easy for a competitor to target a small gym by signing up several new accounts, paying for a month or two, then emptying all the bank accounts at once, causing transactions to fail. If the gym has 1.1K MRR and charges $100/mo per subscriber (that's what my private gym charges for open gym w/o personal training), you're talking about ~11 transactions per month. Failing 3 out of 11 transactions (27%) would probably trigger automated fraud systems.
Stripe customer support is beyond useless, so I totally get your frustration, but I would encourage you to get to the bottom of this and verify that that your "insufficient funds" theory is correct by calling the customers with the failed payments.