Hmm, I haven’t see anyone passing GTM data into a HubSpot iframe before to be honest, however it’s not out of the box supported. So I would be confident it’s to do with the way you’re implementing this customisation.
It’s difficult to say without being familiar with your setup, but I would assume it’s a race condition of some sort where your customisation is passing data to the iframe after the form has loaded within the iframe so the form isn’t getting the data on load.
From my experience this isn’t something that would be supported at all, however if you have developed a custom script to pass data into the iframe I’d look into this code further and how it interacts with the iframe to troubleshoot further.