Vendor keeps saying port is closed but its showing open and I can connect?

Hey,

Our vendor keeps saying X TCP port is closed and they can't connect. We have a cisco ASA 5506.

I've tested by setting the source to any and I can telnet on the specified TCP port from multiple locations....

Canyouseeme shows the port is open

ISP confirmed their IP is not blacklisted

Im not crazy right? If I can telnet in on X port from multiple locations and port websites show its open, there's nothing more I Can do on my end?