I'm going with "all the above".
In the best of times, getting a website or any system to reflect actual physical inventory is extraordinarily difficult. You're at the mercy of the people working with the inventory, and even if you rule with an iron fist, it takes a LOT of human time.
If said inventory is public-facing then you have shoplifters and other miscreants doing unknown things, even little old ladies moving things around the store when they change their minds.
And yep, COVID-19 has screwed with supply chains in some of the most bizarre and unpredictable ways. Panic buyers have stockpiled years of toilet paper and are now snapping up all the freezers. All kinds of commodities and supplies have been re-routed.
And yep, shipping is an overloaded mess all over the planet.
And yep, at the same time it's true that a lot of places are using COVID-19 as a handy excuse for their ordinary incompetence. It can be impossible to tell the difference unless you have history with them.