Strange CPU credit balance report

My t3a.small instance credit report appears to be broken. Setting the CPU credit balance graph to 30s resolution reveals strange phenomenon with lines going from about 0.36 credits each 5 minutes to 576 credits one minute later: https://snipboard.io/YpX8tK.jpg

This also affects our own CloudWatch credit check implementation rendered in Zabbix: https://snipboard.io/45GR9i.jpg

I've already tried to reboot the instance, but the issue persists.

Thanks for help.