how is this distinct from #10951?

Hello @patmmccann , all my setup is similar with fix from previous issue which you attached, but still it is not showing the creative on html page. Previously, when I did the fix it started showing the creative, but now I am trying to show up the creatives for different ad sizes, and it is not working. I do not see any issues at least from set up perspective from our side, that's why reached out to Prebid team if they can help identify the issue. Thanks.

https://qa.vianttech.com/2114_pixie_test_page/hello_world_viant_4.html seems to work now. Is it still an issue @skapoorViant? (if the page was using a new GAM creative / line item, it can sometimes take hours to start working properly).

@dgirardi I happen to find an alternate way to make it work from prebid documentation, but with my old setup it did not work. If you compare these two test pages, the one with _old is similar to my last implementation, just changed the creative size in it, and it did not work. Here are the links below:
[https://qa.vianttech.com/2114_pixie_test_page/hello_world_viant_12_old.html]
[https://qa.vianttech.com/2114_pixie_test_page/hello_world_viant_12.html]

From here it looks like an issue in GAM setup - it's serving an empty creative:

I think this means that none of the line items / creatives for the slot match the targeting.

@dgirardi Not able to understand what does GAM has to do with all this? We are technically using Prebid as SSP and based on the implementation it should be able to render ad with any creative size. And in that test page I am only using one bidder so it should technically always win the bid and render the creative.

If you don't want to involve GAM, you shouldn't be using it (which is what happens in the version that works, which renders directly through Prebid). The version that doesn't work is set up to submit the winning bid as targeting to a GAM slot:

googletag.defineSlot('/19968336/header-bid-tag-0', [[300, 50]], 'div-gpt-ad-1460505748561-0').addService(googletag.pubads());

and

pbjs.setTargetingForGPTAsync();

that only works if GAM account 19968336 has a slot header-bid-tag-0 that is set up to render Prebid bids when appropriate. Please refer to Google Ad Manager with Prebid Step by Step.

Mystery solved, thanks @dgirardi