Firebase Auth Email Link Sign In Broken

I've got a problem with the setup of my firebase account that I need some help with and I'm starting to think this may be a bug. Here's the background on my setup:

• Currently using the following auth methods: email/password, email link, google oauth.
• I have my email/password email templates (resest pw, verify email, confirm email change)set up in firebase to use a custom domain, lets call it user.mydomain.com
• At user.mydomain.com I have a custom app using the firebase js sdk to custom handle the email actions. This is all working great, no problem there.

Here's where it goes wrong:

Prior to setting up the custom domain for my email/pw emails, the email sign in link used the standard firebase app url as the dynamic link and this all worked fine. Now after setting up firebase auth to send the password reset emails through my custom domain, it is also sending the sign in link through this same domain. Because fo this, the sign in link will not function because it does not go to the correct dynamic link to apply the action code and follow on the to continueUrl to finish log in. This seems like a bug because if firebase is confirgured like this, then this should never work once you config a custom domain.

What I've tried:

I've tried seting up a dynamic link in the firebase console (mydomain.page.link) and hardcoding the front end to send through this link when sending the sign in link. However, it always send with the user.mydomain.com suffix vice the mydomain.page.link suffix.

I've also tried to handle the action code at the user.mydomain.com app using firebase.auth().applyActionCode(oobCode). However this invalidates the link everytime and it doesnt seem like there is a way in the firebase js sdk to handle the 'signIn' action.

Has anyone dealt with this? Is there a way I can force just the email sign in links to not use the custom domain? If not, is there a way I can just handle the action code correctly in user.mydomain.app then send the user to the main app?

Any help would be great. Thanks.