You don't need to use your hosting provider for DNS, you can move to something like Cloudflare or other larger DNS providers.
If you are manually entering this value check if the web user interface you are using is converting the value to lowercase automatically. It seems unlikely that the DNS server would be custom software and so it would be expected to behave properly, but their API and web UI will be custom to them and are more likely to do stuff that's wrong.
[ I would advise you reply to their support ticket and say "Please immediately escalate this to a technical specialist for DNS. DNS TXT records must support case sensitive values" ]