If your Paypal webhook / Sycn / Integration is not working correctly or acting weird, then check the following:
1) Check the Paypal is added as a License option on the merchant facillity and the license is not expired
2) Check paypal Setup with correct URL (from the payment form) and all required Event Types specified in the setup article.
Please see note in green information box (above link). Therefore, check that one forms is being used for the paypal webhook setup and others do not have the webhook secret key as well.
3) This could be due to the base site URL on the Merchant facility not being correct. This FAQ article explains the issue briefly and provides solution which is to update the base site URL on the Merchant facility.
Check if the base site URL is updated correctly following this article. If the base site URL had ‘force.com’ then please do see 3.1 and 3.2.
3.1) Then please note that this updation of base site URL does not not update any webhooks setup with the older URL. As a result, any older force.com site URLs that were still being used- even if indirectly or in cached/linked locations - have now become invalid. This would mean webhooks setup for example stripe, shopify, paypal and payments2us.
3.2) The paypal webhook needs to be reconfigured for paypal setup using the new '.salesforce-sites.com/' URL. Follow the setup article shared above to setup again. Make sure to use correct URL and setup for all correct Event Types
4) Apart from the above please check error logs to see if there is an error relevant to paypal. Also please see FAQ Paypal.