If you are using "WP Rocket" plugin, you may encounter some issues with upsell page when running the funnels. This is due to aggressive page caching (which is a good thing!), but it can be easily resolved by excluding the offer page URI from cache plugin.
You need to exclude offer pages by following these instructions-
Step 1: Note down the slug of your offer page from the general settings

Step 2: Open the settings of “WP Rocket” and click on the "Advanced" tab.
Step 3: To exclude "offer" pages enter your noted offer page slug (from step 1) with special characters as shown in the screenshot: "/offer/(.*)"
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Step 4: Click on "Save Changes".
By default, the upsell offer pages slug is "offer". If you had changed the offer pages slug for any reason get the slug from FunnelKit > Settings > General > Permalink.