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/(.*)"
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 (formerly WooFunnels) > Settings > General > Permalink.