We have had the exact same experience. New customers are able to use the Jetpack unthrottled. We have been a customer for MANY years, and we are immediately throttled. We were told it would speed back up to 4G once the congestion let up, but ours has NEVER gone back to 4G once we hit the limit of our "unlimited" plan.
And, I would add that this is even more of an issue now that my children are home due to school closures and expected to learn online. We already pay an incredible amount of money for this unreliable Jetpack. When I spoke to someone about it today, I was told the only option was to increase my bill by $35 a month for 5 more GB. When my children are required to participate in video conferencing daily, 5 more GB is not going to be enough!