I use IS and Zapier a TON.
I have noticed that Zapier can act a bit weirdly. Most of the time I can get the custom field to work with a tag trigger but sometimes it just won’t load. This usually fixes itself in a few hours or so. I assume this is because the Zapier has to poll for updates. You could try rebuilding the Zap and seeing if they come up.
All of that said however, I have a couple options for you:
You could add in an additional action in Zapier, you can add a Search called “Find a Contact.” This will search for the contact and will 99% of the time find all the custom fields.
Or, you could go with my preferred method that is quicker to trigger everything and is much more reliable.
When you use the Zapier trigger for IS it can take 5 - 15 minutes to trigger the whole process. That is because Zapier polls IS over and over again waiting for that tag to be added to a contact. But, there is a way to use the Campaign Builder in IS to trigger everything instantly AND send exactly the information you need to Zapier. It takes a bit of work but once you get the hang of it, it is super useful. Basically you would use the builder to add a goal for the tag being added to the contact and use a sequence to send an HTTP web-hook to Zapier.
Let me know if you need any help, and I will reply back with a full guide for you!