Could it be that shopify is hyjacking the global asp.net core output caching. So Umbraco is stuck using it.
Gemini suggests,,
Keep Shopify on a Dedicated Redis Client:
Do not register Shopify via AddStackExchangeRedisOutputCache. Instead, have Shopify use a direct, isolated Redis client (like StackExchange.Redis ConnectionMultiplexer) wrapped inside your own Shopify repository service. This keeps Shopify's caching entirely in its own custom data layer without touching the web output cache pipeline.
ps Umbraco Output caching is disabled by default. Enabling it is an opt-in decision, I'm guessing you enabled it?
and docs also suggest enabling redis is also a by configuration..
https://docs.umbraco.com/umbraco-cms/develop-with-umbraco/caching/website-output-caching#shared-distributed-cache-redis
though looks like that is also global asp.net core cache.
Finally if you are using a distributed shopify cache.. maybe loadbalancing, wouldn't you want the same centralised approach for the umbraco content cache?
Might be a million miles off here.. but might also spark something with my ramblings.. 😉