Hi lookandbuyme,
Thanks for mentioning this observation. I had not heard of any issues that would cause this behavior previously, but I looked through a few of your listings and was able to see the Domestic Handling Time listed under the Shipping and Payments tab in some but not others.
I was then able to confirm that eBay does have your Domestic Handling Time on file for the listings it is not showing up in, so it appears there must be an issue on their end that is causing this detail not to show up in some listings.
Ebay requires that a Domestic Handling Time be included with all listings that offer any shipping services other than Local Pickup so, if the Domestic Handling Time was not included with the listings when posted, eBay's system would have rejected them and returned an API error.
I recommend contacting eBay's customer support team for further assistance with this issue if you have not done so already. I was able to find a thread on eBay's Community Forum where some other sellers were discussing this so you do not seem to be alone.
-Mike