This is a built-in chromium feature Brave inherited –
Automatic tab discarding.
This feature is invoked and used when a non-active tab(s) (that is, a tab not currently in focus) as a site that’s particularly resource (in this case, memory [RAM]) heavy. The browser will sometimes purge these tabs from background memory in order to conserve resources. Exactly when this type of background data is purged depends largely on the site(s) loaded and the total amount of available resources at the time.
Unfortunately, there is no built-in resource to disable/alter this behavior. There are extensions that will do this for you that you, but that’s the only workaround I know of at this time. Apologies for any inconvenience.
Hope this helps!