Webhook Fails: “The end time must be a date and time with timezone offset” (ISO8601 error)

I’m trying to use Zapier to send new Acuity Scheduling appointments (booked via Squarespace) to my GoHighLevel (GHL) calendar using a Webhooks by Zapier POST request to the GHL API. My workflow is as follows:

1. Trigger: New appointment in Acuity Scheduling.

2. Format: I use Formatter by Zapier to convert both the start and end times to ISO 8601 format with the correct timezone offset (e.g., 2025-05-09T10:30:00-05:00). The output from Formatter looks exactly like this, with the colon in the offset.

3. Webhook: I send these values (along with other required fields) to https://rest.gohighlevel.com/v1/appointments/ with the proper API key in the Authorization header.

Problem:
No matter what I do, I keep getting this error 

“The end time must be a date and time with timezone offset. ex: 2020-10-25T10:00:00-11:00”

• I have confirmed there are no extra spaces or hidden characters.

• The startTime and endTime are different and both are in the correct format.

• I have tried hardcoding the value in the Webhook step and still get the error.

• I have removed optional fields like selectedTimezone to avoid conflicts.

• My Zapier account timezone is set to Central Time (America/Chicago).

Example of my Webhook data:

json

"calendarId": "MY_GHL_CALENDAR_ID",

"title": "Cleaning Appointment",

"startTime": "2025-05-09T08:30:00-05:00",

"endTime": "2025-05-09T10:30:00-05:00",

"contact.firstName": "Test",

"contact.lastName": "User",

"contact.email": "test@example.com" 

What am I missing? Is there a Zapier-specific issue that could be causing this, or is there something else I should try?

Thank you for your help!