The important thing is to compare this metadata:
simpay_charge_count: 1
simpay_charge_max: 3
With the actual number of Invoices that have been processed.
For example:
There have been four processed invoices, and the simpay_charge_count is still at 1, which means the webhook did not correctly run to increment the count when an invoice payment succeeded.
Pending (failed) webhook event: https://dashboard.stripe.com/acct_1OjQpaKQOSCCRnHm/events/evt_1QK5tAKQOSCCRnHmOtcgNvzb
“No such subscription: ‘sub_1QId21KQOSCCRnHmMh4tAQWi’; a similar object exists in live mode, but a test mode key was used to make this request.”
Can you ask if they have, by chance, added custom code to manage their API keys?
If not, ask them to disconnect and reconnect to the same Stripe account in WP Simple Pay → Settings → Stripe → Account and see if that resolves the webhook handling issue.