Great question — answering as the UphillTech team:
There are actually two different gtag that are appearing/firing at various levels of the site. On the initial page, there is a G-XXXXXXGF8V . On the landing page, there is also a Google Ads and remarketing tag AW-XXXXXXXXXX that fires which is creating a separate _ga cookie on the user's machine.
Full answer by A_Patterson: https://webmasters.stackexchange.com/a/144426 (CC BY-SA 4.0).
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