If you are using the "Hummingbird Page Speed Optimization" plugin, you may encounter some issues with upsell page when visiting the offer pages. 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 the 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 "Hummingbird" and click on Caching menu under it.
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Step 3: Select the "Page Caching" tab.
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Step 4: Locate the option "Exclusions" below.

Step 5: To exclude “offer” pages enter your noted offer page slug (from step 1) as shown in the screenshot: */offer/*

Step 6: Scroll down the page and click on the "Save Settings" button to update the changes.

By default, the upsell page's slug is "offer". If you had changed the offer pages slug for any reason get the slug from FunnelKit (formerly WooFunnels) > Settings > General > Permalinks.