Hydration Error, but Only on Custom Domain, and Only on Initial Page Load

[1b4Pi]

I have an SSR/PWA app deployed on Cleavr. I have two sites connected to this app, deploying from the same branch of the same repo. So, both sites have the same identical code.

The only difference between these 2 sites is that one uses a temporary domain name generated by Cleavr, and the other points to a custom domain (the public-facing production URL). That's the only difference.

However, these 2 sites with the same identical code behave differently. The site with the custom domain shows a Hydration error on the initial page load, but not after refresh. And the site with a temporary domain name shows no error at all.

The custom domain site boots differently too. I have navigation and page data queried on boot. The temporary domain paints this data quickly onto the page. But the custom domain site does not.

Here are some illustrations:

This is what both sites look like booting up for the first load. (Private windows, cleared cache):

This is what both sites look like once the page is loaded. Again, both sites have the same identical code, deployed the same identical way. But the site with the hydration error (right) is on a custom domain:

That hydration error disappears on reload. I can't figure out a way to capture the source of that error with it going away on refresh. And since it only happens on the public-facing domain, I'm not sure how to go about doing a sandbox example(?)

Links to both sites are below. Disclaimer before clicking: Please be advised that this site is for a lingerie brand and some content contains partial nudity.

Temporary domain:
http://vu7vxxwx9h7xququ3466.cleaver.rocks/

Public-facing domain:
https://supernaturallingerie.com/

I can't begin to describe how perplexing this is to solve. Again, the same code on both links is deployed identically, but one has a custom domain. Even Lighthouse test results are different, with the custom domain having a higher cumulative shift compared to the temporary domain.

Any ideas as to what's going on? Many thanks.